I agree with Sneaks, it seems to vary a lot. In my department, the more technical ones tend to be a little shorter and the more social sciencey ones tend to be slightly longer, and they tend to be anywhere between 50,000 and 80,000 words. I think having a maximum limit is useful, though - short and tight seems like a good aim, but covering everything means (unless it really is a discipline that can be very short, like maths) most people are likely to be looking for ways to edit down their thesis, rather than worrying about it being too short. (Or is that just me, being all wordy an' stuff...?)