After whittling down the potential candidates for Microsoft published games that were affected by cancellation rumors—developers confirming that Banjo Threeie, Alan Wake and Fable 2 were all safe—it appears that 1UP may have revealed the title that spawned said rumors. Despite being described by EGM editor Shane Bettenhausen in a recent 1UP podcast as in development for a "very long time" and a product "that people are very excited for", it appears that Marvel Universe Online is the source of cancellation gossip. Yes, that sound your hearing is the quiet shrugging of thousands of shoulders.
Nothing has been shown of the massively multiplayer online game save a pre-rendered teaser trailer since it was announced in September of last year, but Microsoft Game Studios and developer Cryptic clearly had high hopes for MUO. According to 1UP's report, "serious, potentially stalling, development troubles" may be responsible for reps at MS and Cryptic being mum on its progress.
That Cryptic recently sold intellectual property rights to the City of Heroes franchise, losing a portion of development staff in tandem, may have something to do with MUO's rumoured problems.
As of right now, it's all unofficial and Microsoft doesn't seem to be providing any fresh details on the game. Keep your eyes on it though, as Marvel Universe Online's prognosis doesn't sound too promising.