The Official Gmail Blog serves up a useful tip for laconic folks who like to send subject-only messages: add
(EOM) to the end of your subject line to skip Gmail's prompt confirming if you want to send the message without any text in the body. Update: <EOM> works as well, and it doesn't appear to be case-sensitive. EOM, of course, stands for "End of Message" and it tells both Gmail and your recipient that's all she (or he) wrote. Here's more on how EOM makes your email more efficient.