As much as people loved the app, iPod touch owners were left out in the cold. Not now. The app has been updated to run on the iPod touch with OS version 3.0 or above. And if you've already been using it, the app has been improved with more data, a larger vocabulary and some improvements to its reasoning algorithms.
You can also give it integer math problems and you'll get an answer. The app uses Microsoft Instant Answers from Bing for the heavy lifting.
Siri uses the speech recognition from Nuance Communications, which also powers the Dragon Dictatation and Dragon Search apps for the iPhone and iPod touch. It is uncannily accurate in my daily use, so Siri has gotten a bit smarter and learned a few new tricks. For free, it's a must have.