According to Capcom, there are absolutely no plans to release a demo for Marvel vs Capcom 3, their upcoming addition to the Versus franchise.
The news came from a recent Q&A session over at Capcom Unity, and Christian Svensson, Capcom's VP of strategy planning, said "Sorry, but zero percent. As with Street Fighter IV, there will be no demo for Marvel vs Capcom 3."
This is bad news for those who're on the fence, but the game is out soon enough.
Source: Capcom Unity