Nitter is a Perl script I wrote when I wanted SMS alerts from Nagios. However, at the time I didn't have access to an SMS gateway so I utilised Twitter's ability to send SMS messages when a user receives a DM from a friend.
However with most mobile phone OS' now supporting some form of Push Notification, SMS is a bit redundant. Especially if used in conjunction with something like Boxcar where you can selectively send Push Notifications.
Nitter requires the Net::Twitter CPAN module. Full instructions are included in the README.