There are not yet rules for playing a 'full' lycanthrope, though it sounds like those may be available when the Ravenloft game comes out (though that is over a year away.)
However, there are rules for shifters, who are basically lesser Lycanthropes, with rules for the race, feats, and a paragon path found in PHB2. You could easily reflavor those elements to make a character that seems relatively lycanthropic - though the paragon path is flavored more towards werewolf than werebear.
Outside of that... you might check with the house rules forum and see if anyone has come up with something. I can think of several ways to present it, possibly going with the Shifter rules as a starting point and having a chain of feats to represent fully awakening as a Lycanthrope, with a Paragon Path showing you fully embracing that path.