Is Objective-C portable ?

What do you think of when people ask you this question? Do you feel there are barriers to making your Objective-C programs portable? If you could port a Cocoa program to Windows or Linux easily, would you do it? Would you care if you had to do it on a Windows or Linux machine or would it just rock if you could do it all with Xcode on OS X?

I write programs for Windows, Mac OS X, Linux and Solaris, and they are all in Objective-C. I am interested in what other people are doing in this field and if this is a field that interests people. Let me know what you think.