Update: As the comment below says, this answer is obsolete.
As everyone else has said, @require only gets run when the script has installed. However, you should note as well that currently jQuery 1.4.* doesn't work with greasemonkey. You can see here for details: http://forum.jquery.com/topic/importing-jquery-1-4-1-into-greasemonkey-scripts-generates-an-error
You will have to use jQuery 1.3.2 until things change.