My guess is that a "humanitarian" invasion will never happen.
Does Burma have vast oil reserves (like the Middle East)? Does Burma's instability threaten to spill over into some portion of the "1st World" (like the Balkans?). Although I confess a shocking lack of geographical knowlege for this part of the world, I am pretty sure the answer to both questions is, "No." There are no vital 1st World interests at stake in Burma.
The recent efforts to aid Indonesia show that 1st World countries will act on purely humanitarian motives - no vital interests there, either. But Indonesia is an open country, and that openess allowed a tidal wave of shocking video images to flood 1st World TVs and PCs, which inspired most of the aid. I could see Americans supporting aid drops, unapproved by the Burmese government, right up until the first plane is shot down - or just shot at.
Pre-emptive Warfare is bad policy, whether the motive is humanitarianism or hunger for oil. Why do people (especially Americans) continue to leap so quickly for a military solution?