I'm skeptical that this is happening. As mentioned, there's been rumors before about Triple H and Shawn Michaels competing against one another at a Wrestlemania, most notably at 29 this year. Those rumors stemmed from the fact that they butted heads during the "End of An Era" build to Wrestlemania 28. There was even speculation that they would do a one year build starting the next night after 28, but it all turned out to be false (the sources were Wrestlezone, which is among the least credible of sites.)
I doubt it will happen this time either (especially with Bryan/HHH seemingly set for Mania, and I can't see that match happening at any other PPV) but if it did, it would be fitting for Triple H to retire wrestling his best friend one last time. If Wrestlemania XXX weren't so close to HHH's recent heel turn (can't see him going back babyface less than a year later, though maybe I'm wrong), then I'd say that HHH could even turn back babyface after losing the match cleanly to Shawn and then they embrace each other with a hug or some shit. Very similar to the End Of An Era match, only without Taker there this time. (And if this match happened, I would hope and expect it to be a pure wrestling match, just like their classic on Raw in December of 2003, as opposed to a match that would have any sort of stipulation attached to it.)
Also, like others, I would like for Michaels to keep his word because I like finality with things and I like when people actually mean it when they say they're hanging the boots up. Can't say I would be literally upset if he did return for only one match, though.