There are several ways you can do it.
You need one of the following: ipod nano, ipod touch 2G, or the Nike + Sports band.
1. If you choose to use a Nano you also need a Nike+ sports kit. It has a sensor that fits in the Nike + shoes, or if you have different shoes you can wear it on your laces in a "shoe wallet" The other part of the kit fits in the end of your Nano.
2. If you choose the Sports Band, you do not need an ipod at all. The band is like a watch that gives you the read outs & later hooks to your computer to record progress at the Nike+ site. (I believe about $50) This option doesn't allow for music.
3. The new Ipod touch 2G has the built in receiver. THen you just need the shoe sensor. (Again in the shoe or mounted on show with shoe wallet or holder) I have not seen these for purchase alone, but you can buy a nano kit & use the sensor from it. (About $30) Sell the receiver on ebay, someone is bound to have lost theirs.
All options let you download your info to Nike + siste for private personal tracking.
Hope this helps.