I have just tried to play a tournament on a forum I frequent, and I got the "Sorry, you've already played today" screen.
Not only have I not played today, I've never played that particular tourny at all.

Has this come up before, or did I just hit a temporary glitch?

Make sure you log out. You'll need a separate tournament account for each tournament you play.

If you have never played it before, you need to log out, then set up a new 'account' for that tourney. Once you are logged in to that particular tourney, you should be able to play.

The 'you have already played' and the fact that you appear to be logged in, will be because you had probably played a different tourney already today, and are logged in to it.

Did that make any sense?

D'oh!
Should have thought of that! Works fine now, thanks.