Jet Set Radio was supposed to hit PlayStation Vita today, but Sega has hit some bumps in the road and delayed the launch. The reason: "necessary development optimizations", meaning it runs poorly in some fashion or another, and requires some fixing up.
Fortunately, if you have a PC that isn't total trash, a PS3, or Xbox 360, you can play it right now. If you don't and were counting on this version, you'll have to wait some, though as of yet I don't have a new date for you.
More news when it hits.