I'm unfamiliar with iOS - does Apple allow AT&T to shove anything on their phone? (I doubt it)
Apple doesn't allow any of that. They control everything, which is why it's going to be interesting to see the iPhone on Verizon. Apple needs Verizon's network more than Verizon needs the iPhone (especially with the success of their Android lineup).
On that same note though, you can root (jailbreak) the iPhone, but that's about it. No custom ROM's or anything like that. But, at least everyone gets the latest iOS at the same time. Doesn't matter if you have a 3g, 3gs, iPhone 4 or even an iPod Touch.
I'll still take Android any day though, no matter what version my Fascinate is stuck on.