Buried at the bottom of the press release for PlayStation Plus, Sony mentioned that users who sign up for the service will have to install firmware update 3.40 first.
Meanwhile, other than updating the console to use the service, there are many holding out hope that cross-game voice chat will be included in the update.
Here's the excerpt from release:
PlayStation users can purchase PlayStation Plus membership through PlayStation Store on PS3 only. Users need to install PS3 system software version 3.40, which will be released on June 29nd, to enjoy PlayStation Plus.
Start the wondering now.