I updated WordPress to 3.1 last night and as usual it broke a bunch of things. This time it stopped my method for excluding Twitter backup posts from my RSS feed. I'm frightfully sorry, I didn't mean to spam you like that!
I frantically installed Advanced Category Excluder but its also horribly broken. Fortunately, the latest beta of RYO Category Visibility works a treat, and is far simpler than ACE.
Thank you for your patience ^__^.