I read (Prowrestling.com) that plans for Mania now are as follows:
Undertaker v Edge (setup is obvious when you see Smackdown)
HHH v John Cena (with the possibility of a triple threat with Orton, personally I doubt that)
MVP v Matt Hardy (something they've been planning for a while now, Hardy should be back anytime and I hope they give this feud a good lotta tv time, I really enjoy it)
McMahon will either be involved in a match with Finlay or Foley according to the site. It could be Foley/McMahon, Foley/Shane, (or Finlay) or possibly a tag match. Horswoggle may even be in the match, it's all speculation at this point.