Here is a little script that can be used to tweet your current iTunes song to twitter.
Obviously, add your twitter username and password to get this working.

#!/bin/sh
user=""<br />pass=""
curl="/usr/bin/curl"
name=osascript -e 'tell application "iTunes" to name of current track as string'
artist=osascript -e 'tell application "iTunes" to artist of current track as string'
message="Now playing $artist - $name on iTunes."
$curl --basic --user "$user:$pass" --data-ascii \
"status=echo $message | tr ' ' '+'" \
"http://twitter.com/statuses/update.json" > /dev/null
status=$?
echo "Your tweet returned a status code of $status"
exit 0

Thanks to Linux Journal for the curl command and macosxhints.com for the osascript help.