> What's planned to be the next 'stable' release?
All kernel series get 'stable' releases, the next one planned is 3.8.1. ;)
What you probably meant is 'longterm'. While any kernel developer can pick up a kernel series and maintain it as longterm, the stable team (e.g. Greg Kroah-Hartman) only maintains one longterm series per year, which is selected early each fall.
Historically, 3.0 was released July 21 and announced as longterm October 26; while 3.4 was released May 20 and announced as longterm August 20. So the next longterm series will probably be either 3.9 (to be released early May), or more likely 3.10 (to be released about middle of July), but don't expect an announcement prior to August.