I think there is quite a big difference between e.g. weblogs and 'wordy forums' (think: long political/social discussions) on the one hand, and question/answer-type forums (usually short to-the-point posts and replies, with the occasional need for a long uninterrupted line (code, ascii drawings). I think the fluid lay-out suits this type of fourms more, whether typography agrees with that or not
Thanks for restoring the 'fluid option'. If you could make an extra one that makes the width around 75% (now it's 100%, I think), that would be nice.