As for hooking up the iPod to the FACTORY radio, I believe these other gents have covered it. With the options listed however, you lose sound quality, especially with the cassete. And you're never certain wether or not you can find a decent empty radio station for the FM transmitters. I have not heard of the harmon/kardon however. The best option in my opinion, is a direct hardline into an aftermarket CD player. No loss of sound quality, and no tape deck / car lighter junky look.
Certain CD players have a LINE IN on the reverse side of them. Some models as low as $150 have them. There are cables at Best Buy that enable you to hook your iPod directly into the Line In of an aftermarket CD player. This will be slightly "rigged", however, if done well and installed with some time, you can hide the wiring, and be left with an extremely clean look, all the while having ALL of your music with you. I have not done this mod myself YET. However, it is near the top of my list, and with a little know how, can be done fairly easily. Talk to your BestBuy audio guys. If they know anything, they can help you out.
There are also KITS available for certain makes and models of cars, but they can get up to $400. I don't really feel like shelling that out after paying $300 for the iPod alone.