This is the second time I've done this recently and promptly kicked myself. If you're going to leave the domain (shouting to myself)
MAKE SURE YOU:
[A] HAVE A LOCAL USER ACCOUNT WITH ADMIN PRIVILEGES ENABLED
[B] KNOW THE PASSWORD
Twice in the last few months I've tried to connect to the domain with a machine (or OS partition) that I haven't used in a while with no success as the machine account has (I assume) expired. Easy, just leave the domain and re-join! Trouble is it's so easy I've done it before thinking about it. The first time I fell foul of my own policy of disabling all local user accounts as soon as I've setup my domain account on the machine. Then today I just couldn't remember the password for the Administrator account (which, luckily enough, I'd forgotten to disable).
As soon as the login prompt came up with no domain option the realisation set in and a good bout of self flagellation followed. It was painful but worth it...