We've been working on adding Stage3D to our engine, and it's gone very smoothly indeed.
This is only to get the games working on mobile (for the moment at least), as we don't believe there's enough compatibility out there for Stage3D browser games to be successful yet.
I've written versions of the rendering code for Stage3D, not using any 3rd party libs like Starling or Genome, so the overhead is very light. I'm using project specific properties to switch between versions, so it only compiles in the correct version of the code.
I'm creating texture pages in-game, as the graphics are mostly stored as movieclips. This way we can have full quality bitmaps for hi-resolution devices, and drop back to low resolution for older machines. Still waiting for Adobe to add in some texture compression tools though, as currently the only option we have is 32 bit per pixel, which is INCREDIBLY wasteful.
So, we've just started work on SoccerBalls 2, and this will be the first one we've developed to work on both mobile and browser. Looking forward to seeing how well it goes....