One of the major strikes against the Prius that caused me to look elsewhere.

By all accounts, you're pretty much S.O.L. They have an extra-special magic woo-woo Toyota-only radio. You can order an XM adapter over the internet that works with the computer stuff for under $200, though. They even have an installation DVD for it. A web search or two for "Prius XM" should turn it up from multiple vendors.
http://www.priusxm.com/

There are some plug-in adapters that make sound and/or video play in the Prius, but you'll probably still have to mount your MP3 player under the passenger seat and have a wired control brought out to the dashboard somewhere.

You could try tracking down specialty car audio shops in your area and call them up and ask them. I doubt the steering wheel controls and touch screen will work (for the radio) afterwards. Most likely it will cost more than the XM thing to get your existing toys installed and jerry-rigged to operate.

These people have numerous hacks for the Prius, and the number of annoyance-killing hacks was the second major strike against the car.
http://www.coastaletech.com/04prius.htm