I'm really loving this new integrated spell check in Firefox 2.0. I don't get why spell checks are not integrated into OS's at this point. I mean, you would think that with Vista and Leopard or Cheetah or whatever is coming out of Apple would have this feature.

Microsoft uses their spellchecker to add value to Microsoft products. If they made it a feature of the OS then every program could use it and that would diminish the relative advantage of their products.
There are tonnes of libraries for spell checking, so no one feels a strong need to include one as part of the OS. Aspell does ship as part of many Linux distros, though

OSX has a built-in spell checker. It is very handy to have one dictionary across all applications, and to have a dictionary in nearly every application, without additional developer effort!