Of course, you can write Objective-C code in notepad or other programs and then move it to a Mac to compile.
But seriously, it depends on whether you are developing official applications to put in App Store or developing applications for jailbroken iPhone. To write official applications, Apple iPhone SDK which requires an Intel Mac seems to be the only practical way. However, there is an unofficial toolchain to write applications for jailbroken iPhones. You can run it on Linux and Windows (using Cygwin).
