For those of you wondering (and not already discussing the news in the Kotaku comments section!), yes the PlayStation 3 included in the Metal Gear Solid 4 bundle announced earlier today will play select PlayStation 2 games via hardware and software emulation. SCEA PR manager Al De Leon confirmed that the hardware is similar to what was included in the 80GB Motorstorm bundle and will feature partial backward compatibility. While the original Metal Gear Solid for PlayStation should play without a hitch and Metal Gear Solid 2 is listed as having "no major problems", you may experience a hiccup or two playing the third MGS game.
According to the official PlayStation 3 backwards compatibility database, Metal Gear Solid 3 has the following issues: "At various points throughout gameplay, the title performs at a significantly slower than normal performance speed" and "During various FMA sequences throughout the title, the FMA does not play as 'smooth' as normal." Not bad. Not bad at all.