I think this year will be his final "active" year, and he'll retire next year. I'm not sure if it'll be at WrestleMania because Undertaker could be retiring next year, and both Edge and Taker retiring on the same night isn't likely IMO.
His first PPV was SummerSlam, wasn't it? It's the second biggest night of the year; could retire there. I mean, they'd have to break their Staples Center streak and take it to Canada to make it a big enough deal to justify his retirement happening there, but it could be done.
Then again, if they want Undertaker to retire putting someone over, they could retire him at Survivor Series 2011 instead since he's in such bad shape, and have him take the PPV with him as they're trying to get rid of it. He'd get to lose, and be officially gone from the active roster with a send off and everything. You plant the seeds with someone badmouthing his send off or somehow disrespecting him, then have him make a big one night only triumphant return at WrestleMania to go 20-0.
If they want to do it right without having a double retirement, they could easily make WM 28 Edge's retirement night and have Undertaker as a one night only attraction. Sure, it'd be a shame not to have Edge go out in Canada, but WM is big enough either way and, if he's on SmackDown at the time, they could feasibly do a special, live, post-WM SmackDown or something in Canada so he does get to say his goodbyes there.