I think there's cause for legitimate concern over Taker losing his streak to Cena at WM30. It's wrestling tradition to go out on your back, and Taker seems like the kind who would respect and adhere to that tradition. But on the flip side, WWE sees Undertaker as such a respected legend that they may make an exception in his case (depends on which they feel more strongly towards - desperately trying to make Cena their greatest star ever or honoring Taker's legacy at Wrestlemania the best way possible.) They even put Taker on their version of Mt. Rushmore, even though realistically, that fourth spot should have gone to either Bruno or The Rock.
That's why I'm all for Cena winning the belt at Wrestlemania and having a long title reign and dominant year, because it increases probability that he loses at Wrestlemania next year.