Remember, whatever is on the "T-Mobile G1" is the T-Mobile customized version of Android on the T-Mobile customized HTC Dream. It will not be a generic Android experience. What I heard was that you needed a GMail account for installation/registration. It's possible that's just because the mail client that ships is GMail-only, or some other non-obvious reason. I haven't seen anybody say the end user will be limited to apps from their proprietary store either, so I assume if you don't like the built-in app(s), you can just replace them (unlike the iPhone).