You could use Python's SimpleHTTPServer module and implement your own SimpleHTTPRequestHandler that will do the SMS sending. The advantage of this approach is that you could even send SMS from your computer by pointing your browser to the N900's IP and a custom port when your N900 is connected to your local home network. (of course, for security reasons, you can also opt to only listen on the loopback interface, so that connections from outside are not possible)

Have you tried VertSMS already?