As someone who is currently developing a game for the iPhone / Android using Flash, I can tell you that it will be a tough battle. I can create a game that runs at 200+ fps in a browser (although I would never run it at that speed) that only works on 20-24 on an iPhone. And we are talking about an extremely, extremely optimized game that implements a different rendering engine for iPhone and Android.
These devices are so weaker that the package was applied in a general way that it worked for everything that you created in Flash, but does not fully use the GPU rendering capabilities on iPhone or Android. Perhaps when Molehill comes out, it will be a different story.
With all that said, if you create a puzzle game or a more basic application, everything will be all right. If you create something much more intense, such as side scrollers and / or a lot of animation and physics, you will be very disappointed.
Good luck