15 will launch "very soon." As rumoured, the update includes the ability to play PSP Minis on the Playstation 3. If you've already bought some Minis for your PSP, worry not, you'll be able to download them again to your PS3 and enjoy them on the big-screen.
The other big feature of Firmware 3.15 is the ability to data transfer between PS3s. The transfer tool makes it super easy to copy content between systems, and is presumably being introduced as an added incentive to get current PS3 owners to upgrade to the PS3 Slim.
You can read more on the European Playstation.Blog.