How are you going to charge and play music from the iPad? Are you running a separate charger cable and then using the music from a headphone jack or are you charging and playing through the apple cable? If so, what headunit did you choose? I want to charge and control my iPhone from the radio but the only headunits I've found make you relinquish control from the phone when you use the apple cable.
The good thing is that I can relinquish the phone functions on the ipad being that there are none ;D There are plenty of compatible units. I will not be using the usb powered cable but one that comes as a kit with the head unit.
Currently Im still using the super cheap JVC, but I have my eye on a few head units. I just have not made up my mind yet as to which one to use. Most probably it will be a kenwood.
As far as iphone compatibility, there are quite a few of them out there that will still allow you to make and receive calls. For the most part you have to buy the manufacturers connectivity kit. Also keep in mind of the variations with the iphone. Many that are 3Gs compatible will not accept the Iphone 4. I would just call crutchfield or the manufacturer to see if they have the firmware updates.