The Tweet (Help):
"Why 140 Characters?
140 characters is the maximum length of text messages sent using SMS. Twitter integrates well with mobile devices. To tweet from your phone, send a text to “40404”. Also, the hundred and 40 character limit encourages rapid sharing, it is easier than writing a blog post for instance. The true maximum payload of SMS can actually be 160 characters, but twitter reserves 20 characters so you could put in the username for @replies."
@twibes Why is a Tweet 140 characters? http://j.mp/cvskon