[cdh]

Someone can correct me if I'm wrong, but I believe the language itself can be used on almost any platform with gcc, not just OS X.

[octotoad]

You are correct. GNUstep would be the most well known project to make use of ObjC outside of the Apple world. Also, since it implements the OpenStep API, you could use it to get a basic feel for Mac development without jumping in to the Apple fog.

[Turing_Machine]

You're quite right, though of course you don't get the Cocoa libraries. GNUStep might give something of the same flavor, perhaps.