The ultimate match that's left for WWE to do with the Undertaker is him vs John Cena. That's a big marquee match that would fit right in at Wrestlemania 30 (with it being an anniversary WM and all that).
So I think Taker beats Cena at next year's event, then retires for good.
Has he still got it? No, he hasn't. He's obviously not as good as he was a few years back. However, he's still somehow managed to put on great matches against HHH the past 2 years (even if both have left him in rough shape). I still think that with 12 months between each Wrestlemania, that Taker gets enough time to rehab his body enough so that he can work one match each year. And next year, I think should, and will be his final one.