According to Sony, Cross-Game Chat will be a new feature on the PS3 with Firmware 3.40 is available to download later this month. The confirmation about cross-game chat first appeared on the Swedish PlayStation Website, but was later taken down. It appears that cross-game chat will be part of PlayStation Plus.
Detailed on the website:
# Cross-game Voice Chat: invite your friends to chat online no matter what game you’re all playing, even if they aren’t members of PlayStation Plus.
(Note: This message is also on the PlayStation Swedish site. Update: It has now been taken down. )