I upgraded my self-hosted blog yesterday and it was simple. I used auto-install and it took maybe 2 minutes, Sweet.
As always, back up the database and disabled all plugins before upgrading.
I suggest using the the special WordPress Memory Bump plugin as a precaution. It extends the amount of memory the install program can use when upgrading to 3.0. De-install it when done. It prevents a memory exhausted error that a few have gotten.
For such a major upgrade, 3.0 has been remarkably trouble-free. A few plugins have problems, that’s all, plus that memory error. Check the WordPress Install Forum for further information.
Well done, WordPress.