NilObject is right that you can only develop for the iPhone officially on a recent (last few years) Mac with OS X.
That said, Objective-C isn't tied to the iPhone. You can write programs for your PC in Objective-C, which would give you experience without needing the iPhone and Mac. Objective-C is a standard part of the GNU Compiler Collection (GCC). It's rather easy to install with Cygwin.
If you really want to make iPhone applications, you should consider a Mac. You'll get all sorts of experience and can also program for OS X then. Since laptops were just released, many people are selling their previous models because they have upgraded. If you can find an old Intel chip based Mac Mini, it won't cost you much at all (a few hundred), but it won't be fast. The recent Minis haven't been updated in a very long time and are not a very good value anymore for their new price (in my opinion), but they may be available cheap too.
