According to a CVG report citing a "development source," PS3 firmware 2.5 will include an option for screen capturing, allowing users to take a direct-feed shot of what's currently running on their PS3 - either in-game or from within the XMB.
Seeing as development firmware for both PSP and PS3 has had a built-in screenshot function for some time, file this one under probable, but aside from this rumor - there's been no other indication that it will show up in firmware 2.5. We'll see.
First PS3 firmware 2.5 details [CVG]