The skills are laid out in the chapter named Skills in the PHB. There is no Tumble skill. The two closest things are acrobatic stunts ( which do not allow for avoiding OA's) and the Rogue utility power called Tumble.
There are a variety of other powers that allow for movement on the battlefield without provoking.
Yup Tumble is now a 2nd Level Utility power useable 1/encounter that allows the rogue to shift at half their speed. Artful Dodger Rogues gain charisma modifier as a bonus to their AC vs OAs. Halflings receive a natural +2 vs OAs.
Shift one square as a move action. Some of the rogue powers allow you to shift foes and yourself around a bit but not much.
The question here is: what do you want out of Tumble? You can probably find skill-based uses in Acrobatics. If you want to negate Opportunity Attacks, you'll have to rely on powers (like the Rogue's Tumble power).