Hi, this is Seth A. Roby (the author).
The answer is, as most answers are, more complicated than a simple yes or no.
Objective-C can be compiled by gcc, which means that you can use it pretty much wherever.
However, as Retro says, the main strength of Objective-C in OS X is the frameworks. But Retro's a little off when he says that those are available only fro OS X.
But OS X is not where these frameworks originated; for that we have to go back through OPENSTEP, where NeXT and Sun worked together on the frameworks and made them an open standard, to NeXTSTEP where they were originally invented.
And OS X is not the only place that took the frameworks from OPENSTEP; GNUSTEP (gnustep.org) is making an OPENSTEP-compliant code base that can be used by anybody (it's even under the friendly Lesser GPL). GNUSTEP works pretty well (I'm told) for Foundation (low-level) classes, but the AppKit support is spotty at best. GNUSTEP works pretty well on Linux and newer FreeBSDs, but elsewhere they generally report themselves as unstable (see http://gnustep.org/information/machines_toc.html). The big deal, though, is that GNUSTEP is going to be playing catch-up with Apple as Apple extends the classes available, which Apple has been doing and GNUSTEP has pledged to follow to the best of their ability.
So the short answer is 'yes', but the longer answer is 'mostly.' Objective-C is very much available, and the frameworks are making their way there, but the support is not quite there and the latest and greatest will always be OS X only. Unless, of course, Apple re-releases Yellow Box for Windows. I expect that sometime after the return of Enterprise Object Frameworks, which should be sometime after armageddon.