I'm sorry, but I'm a little confused. Are you saying that windows mobile is losing market share? Every new cool phone eclipses the old cool phone. windows mobile on the AT&T tilt smashed AT&T's 8525 phones. And, right now the iPhone is smashing the windows/android based devices... but as we speak the tide is turning. the first and second wave of iphone users now have their 2 yrs contracts firmly in place, and it's other devices that will start to take back market share percentages. come what may, windows mobile is not going away
now as to the whole software distribution. If you have a MacOS, Windows, Linux, etc software you built in your basement and wanted to start selling, you will consign it to someone, or build up a webpage and market the darn thing yourself. With Iphone, you don't have that option. With android and windows you do. If company A is big enough to secure exclusive rights on the iphone store, you're out of the game anyway. I think your argument about software distro is a little off, but i'm willing to listen to your side again.
it's a shame that t-mobile didn't advertise the G1 better. I understand that they didn't want to emphasize android, as soon you'll be able to get a phone with that from any vendor... but soon the world will wake up to android as a development platform, and then the really cool geeky projects will come out. Android has a market place, and yet allows direct installs. I'm really happy with it right now.
In answer to the original question, i feel that the correct platform to build your home brew app on is windows mobile. if you are anti microsoft enough to refuse that... then go with android. the market there will catch up (in case your ever thinking about going commercial with your product).