The only reliable info I could find is at the always-excellent MetaFilter
http://ask.metafilter.com/110466/Anyway-to-develop-iPhoneiTouch-apps-without-investing-in-a-Mac
The answer is apparently no.
You absolutely need an Intel Mac of some description.
The entire iPhone build process is too deeply ingrained in XCode to build elsewhere; and the only other Objective-C compiler I know is gcc, which doesn't support any Apple's additions to the language (nor their libraries).
And, in direct opposition to what people are saying above, Objective-C is absolutely my favorite native, compiled language. Elegant, small (only a few changes from C), late-binding, dynamic, straightforward. It's what C++ should have been.
Lots of people recommend picking up a secondhand Intel (remember, must be Intel!) Mac Mini as the cheapest "port of entry".