One of my websites sends SMS messages when "interesting" things happen. After switching to AT&T for wireless, I had to rewire a bit of code to cope with AT&T's requirement of presenting a session cookie along with the POST to their pager CGI. The rewiring took 20 minutes, including the time it took to find my copy of the Perl Cookbook, which has an example of using HTTP::Cookie to hold session cookies between requests. I love Perl.
I posted a demo script of this over at Perlmonks. Since I wrote the original code a few years back, several SMS packages have showed up on CPAN.