It depends entirely on what you mean by getting some real work done, it'll boil down to the apps you need basically.
OpenOffice with the latest version I think has come a long way.
If you want to develop .NET apps in Linux,
Mono is what you want. Though really 99% of .NET apps are for Windows, so it makes more sense to develop there I think. And I wouldn't dual boot, I'd run Windows in a virtual machine (if I wanted Linux as my primary OS).
No idea with MobileMe with Linux, but isn't that just browser based? Wikipedia says Firefox 3 works on Linux with MobileMe with a warning. Or you could maybe use Wine to get Safari running?
The latest version of Ubuntu just came out and it's probably the closest thing to a real primetime desktop OS, and its come a long way but I still wouldn't say it's for the general public.