And the htc and other android phones all run the same OS (different versions; htc phones run 1.6 and most others run 2.1). They have a different ui layed over them though. Htc has their sense ui, and almost all others use the stock android UI.
Android is so fragmented that it will eventually fall to shit. It will be like windows. Everybody will have it, but nobody will like it.
Dont get me wrong, it's an awesome OS, but it's already fragmented right now. They aren't even all running the same version. A ton of them are running a very outdated version.