Well, like I did way back with 2.80/2.81 and 3.30, I've dug through all the module files in firmware 3.71 to see what I could find.
Here's what I've found hidden in 3.71 so far. Take away from this whatever you want; hopefully it'll stir up some speculation and get people interested.
* An official screenshot plugin, which saves either JPEG or PNG images to ms0:/PSP/SCREENSHOT, named *_####.JPG or *_####.PNG
* Hints at an official web-based content service, possibly for buying music/videos
* An Xbox Live style network gaming service, including the browser useragent strings related to it (PS3Community-agent/1.0.0 and PSP NP/1.0), using an XML-based method of communicating with the central server
* When you stop viewing an image and return to the list, the image fades out instead of disappearing.
* Custom themes are saved to flash1:/vsh/theme/custom_theme.dat . The 3.71 fw comes with the 'cookies' theme preinstalled, saved as flash0:/vsh/resource/custom_theme.dat .
* The web browser reports the PSP's version as 3.70.
* Registering your PSP with a LocationFree base station makes a file called ms0:/MSSONY/LF/REGISTER/REGISTME.ERD (??? not confirmed, just a guess).
* The HTTP user agent for the LocationFree connection is "LFX HTTP Tunnel Client 1; model=LF-PSP, modeltype=6, generation=3, support=1/2/3, codec=0/2".
* The exact Flash version is "Shockwave Flash 6.0 r72".
* Licenses for DRM-enabled video will be stored as "/PSP/LICENSE/*.PNL", and the video *might* be stored as "ms0:/MPE_ROOT/100AEV01/*.MLV".
* There's a reference to "ms0:/MP_ROOT/105ANV01/MAQ00991.MP4' for ... some ... reason.
* The following game IDs listed in umdcache.prx, for whatever reason:
** UCAS-40025 (Ghost in the Shell: Standalone Complex - Territory of Hunter [Asia])
** UCES-00152 (Go! Sudoku [Europe])
** UCJS-10001 (Mina no Golf Portable [Japan])
** UCJS-10036 (Kazuo, a.k.a. Go! Sudoku [Japan])
** ULAS-42067 (Unknown [Asia])
** ULAS-42096 (UEFA Champions League 2006-2007 [Asia])
** ULES-00147 (Madden NFL '06 [Europe])
** ULES-00468 (Every Extend Extra [Europe])
** ULES-00599 (Star Wars - Lethal Alliance [Europe])
** ULES-00647 (Capcom Puzzle World [Europe])
** ULES-00699 (UEFA Champions League 2006-2007 [Europe])
** ULJM-05087 (Untold Legends: Brotherhood of the Blade [Japan])
** ULJM-05170 (Legend of Heroes, a.k.a. Eiyuu Densetsu - Sora no Kiseki FC [Japan])
** ULJM-05185 (Akudaikan Manyuuki [Japan])
** ULJM-05254 (Crisis Core: Final Fantasy VII, 10th anniversary bundle version [Japan])
** ULJM-05275 (Crisis Core: Final Fantasy VII, solo version [Japan])
** ULJS-00060 (Daito Giken Koushiki Pachi-Slot Simulator Yoshimune Portable [Japan])
** ULJS-00113 (Disgaea: Hour of Darkness Portable Tsuusin Taisen Hajimemasita [Japan])
** ULKS-46015 (Untold Legends - Brotherhood of the Blade [Korea])
** ULKS-46076 (Unknown [Korea])
** ULKS-46116 (DJ Max Portable 2 [Korea])
** ULUS-10024 (Madden NFL '06 [USA])
** ULUS-10095 (Go! Sudoku [USA])
** ULUS-10147 (Every Extend Extra [USA])
** ULUS-10184 (Coded Arms - Contagion [USA])
** ULUS-10217 (Capcom Puzzle World [USA])
** ULUS-10221 (UEFA Champions League 2006-2007 [USA])
* audio.prx refers to ULJS-00019 (School Rumble: Nesan Jiken Desu [Japan]) for... some reason
* isofs.prx refers to:
** ULES-00123 (Rengoku - The Tower of Purgatory [Europe])
** ULJM-05006 (Rengoku - The Tower of Purgatory [Japan])
** ULJM-05063 (SSX On Tour [Japan])
** ULJM-05127 (Dragon Quest & Final Fantasy in Itadaki Street Portable [Japan])
** ULUS-10013 (Rengoku - The Tower of Purgatory [USA])
** ULUS-10042 (SSX On Tour [USA])
* utility.prx pays special attention to all the different regional versions of Tekken 5 Dark Resurrection. It's been suggested that this is because of the way the game uses PSP-specific savedata.
* it also pays attention to:
** ULES00016 (Darkstalkers Chronicle: The Chaos Tower [Europe])
** ULJM05005 (Vampire Chronicle: The Chaos Tower [Japan])
** ULJM05026 (Star Soldier [Japan)
** ULUS10005 (Darkstalkers Chronicle: The Chaos Tower [USA])
** ULUS10021 (Midnight Club 3: Dub Edition [USA])
* Some interesting registry entries:
** /CONFIG/NETWORK/GO_MESSENGER
* References to files that don't exist, either because they're not made yet or because they're for the devkit only:
** flash0:/kd/npdrm_tool.prx
** flash0:/kd/usbserial.prx
** flash0:/kd/usbgps_serial.prx
* lfatfs.prx refers to flashfat4, flashfat5, lflash0:0,4 and lflash0:0,5. These devices don't exist on the slim; at least, I couldn't assign them as devices myself. Attempting to do so returns error 0x8001000D.
* loadexec_02g.prx refers to a new execution mode - 'umdemu', to go along with game, vsh, and updater.
* mpegbase_260.prx refers to ULJS00006 (Soukyuu no Fafner [Japan])
* np_auth.prx refers to all sorts of weird stuff, like:
** the words "Lediatio Lunto Ritna"
** something called X-I-5, with a version of 2.1, which appears to be related to submitting 'tickets' to
https://auth.??.ac.playstation.ne t/nav/auth; probably related to logging onto the network play service
* the pops plugin uses a PS1 BIOS called 'System ROM Version 4.5 05/25/00 J', which is apparently the same as the 3.70 one (as indicated by the partial SVN path 'branches/pops-370/src(r5808)' in pops.prx)
* both umdman.prx and umdcache.prx contain the entire UMD_DATA.BIN file from ULJS-00001 (Ridge Racers [Japan])
* content_browser.prx appears to be some sort of convoluted multifunction plugin... I've found references to widgets/MusicAlarmTime.cpp which implies some sort of musical alarm clock, but also to what appears to be a music player (maybe) including a list of compatible formats:
** msgmusic_info_codec_atrac 3
** msgmusic_info_codec_wav_l inearpcm
** msgmusic_info_codec_1.4m
** msgmusic_info_codec_mp3
** msgmusic_info_codec_aac
** msgmusic_info_codec_wma
** msgmusic_info_codec_unkno wn
** msgmusic_info_codec_atrac 3plus
** msgmusic_info_codec_5.1ch
** msgmusic_info_codec_3m
** msgmusic_info_codec_4.6m
** msgmusic_info_codec_wav_a dpcm
* references to /PSP/APP, which is also created when you run the 1SEG player (downloadable apps, anyone?)
* references to /MSSSJ/GUIDE and /MSSSJ/PE ... whatever they are
* as before, references to /PSP/MINIGAME
* new registry categories:
** /CONFIG/NP, containing keys named account_id, login_id, and password
** /NETWORK/GO_MESSENGER ... containing unknown keys
There's probably a lot more that I've overlooked, but this is the stuff that caught my attention.
There are 
