I had just mentioned compiling MXML without using FlexBuilder and did a quick Google on it to see the current state of such integration. Wow this is pretty cool. 😛 Some neat info and projects in the comments as well including FLit.
I made a few minor changes to the MinimalComps project from Keith Peters and got it playing nicely with MXML. The below application compiles to a tiny 23KB yet makes use of MXML and even data binding.