I have the same question.
I think the best way to develop is to use the Android Dev Phone because it's hardware-unlock.
Besides, it's possible to flash it with new Android version, at least until 1.6 (see http://developer.htc.com/adp.html#s3
). I think commercial phones can not be flash by user...
I would like to add some questions :
Q1 : Do you know if Android 2.0 will be available on the Android Dev Phone 1 ?
Q2 : Does Android Dev Phone 1 have the hardware capabilities to support multitouch ?
I say that because multitouch has just been implemented in Android 2.0 and the Android Dev Phone was release when it didn't exist yet.
Q3 : Is it possible to flash Android phones buy in a market ?