The iPhone SDK is MacOS X only. Don’t see any information about a time line for supporting other development platforms. I’ve submitted a question about this to ADC, and will report back if and when I hear something.
Since the SDK is a modified XCode environment (not sure how one would go about automating builds and integration testing on a build server…) and porting XCode to another platform would be a rather large effort, I don’t expect anything soon.
Not that the iPhone SDK is the only mobile platform sdk that limits what system I can develop on, but the common platform for all other mobile development systems is Windows.
As a result development for the iPhone will come from Apple desktop developers, and larger mobile development shops. Independent developers in the mobile world may be turned off, which is my personal reaction.