I never knew you could send an SMS to a standard landline.
You just 'text' from your mobile to the other number, then about ten seconds later the phone will ring. After a little trumpet sound, a digitised voice speaks out the message including ;) smileys - clever stuff.
The text to speech reminded me of the captcha when signing up for an MSN Hotmail account, where you can listen to the letters being read out as well; useful for accessibility and visually impaired users.
Also Jay has a simple walkthrough showing how to output text-to-speech (TTS) to a .wav file in C#, so you could combine it with this CodeProject ASP.NET Captcha control article.