Depends on what model prius you purchased. Try plugging in your phone, if it works you can, and if it doesnt work you cannot. Theres always the option of installing an aftermarked stereo that is compatable to the iphone, if yours is not already.
I had a 2007 pkg 3 and did this all the time. i just got a $3 adapter off eBay forever ago and never had any real issues.
I bought a 2007 Prius earlier this week. Right away, I purchased an aux cable ($14 at the Source). Simply plug one end into the headphone socket of your phone, and the other end into the aux input (centre storage). Press the aux button on the stereo .., and voila, it plays your music. Cheap and easy. I like it.