It seems that version 2.80 has just come out and it is not a mandatory update. This means folks can still go online and use the Playstation Network and related features without needing to upgrade first.
As for the changelog, it's nothing spectacular, just one sentence:
The playback quality of some PLAYSTATION 3 software has been improved.
The file size is 142.70MB and can be downloaded via the PS3 or a computer via a memory stick (USB or Pro/Duo) from Sony's website.