140 characters is a low number. And even if it wasn't, isn' t it one of our main points when trying to attract tweeters, t <--140 character limit!
... attract tweeters, that they can write much longer posts?
I could live with a 280 limit, just to make posts twice as big as over on twitter (that'll show'em!), but I also think marc has a point when suggesting a 500 limit.