One thing i really enjoy about Twitter is that the limit of 140 characters really forces you to refine exactly what you want to get across in a tweet. You have to cut out all of the superfluous crap, be concise and often have to rethink how to write what you want to say.
I find that often my longer blogs posts are rather rambling and i indulge in as many words as i think i need to get my point across. One of the things @37Signals are always saying is; 'embrace constraints', so i suspect my posts would be better if i imposed a word limit on the length of any individual post.
My gut feel at the moment is to limit to 1000 words, but i am concerned that this might still be too long. I think my longest post was around 1400 words and the average is less than 1000, so it wouldn't really be a substantial cut.
I have put a poll at the top left of this page and will abide by whatever length gets the most votes.