Because most radios in new cars today are "integrated" into the dash or serve multiple functions, it is becoming very difficult to replace the factory head unit. I recently spoke to one of the installers at Ovation here in Indianapolis and he was not aware of any pre-made adapters to make your aftermarket head unit fit the factory dash. Therefore you will most likely have to have a custom dash face made, and the cost and quality will depend on the installer. Basically, if you want it done well, its gonna cost you a lot.
Also remember very few aftermarket head units will be able to take over the functions of the DIC (Driver Information Center). This includes the programable buttons on your steering wheel, personalization settings, trip odometer and much more. Those that are able to take on these functions would be very high-end units and again, require expensive custom programing. If you're willing to spend the money and give up the DIC functions, go for it, I know there are people out there who have done that and I'm sure they're happy with the results. Otherwise your best bet would be to keep the factory head unit or upgrade to a factory dvd/nav unit. You can find these on e-bay or by running a search of the forums.
The last option I know of is something like JL Audio's Clean Sweep. Alpine also has a similar product but everyone "in the know" that I've talked to says the JL is a superior product. Systems like the Clean Sweep were developed to take the signal from the factory head unit and clean it up and balance all the levels in order to give you the purest possible signal to then send out to aftermarket amps and speakers. The Clean Sweep also provides you with a set of audio inputs so you can add a multi-disk changer, ipod, or dvd player. These are about all the options I know about, try running a search of the forums to see what other information you can find which might help you.
Finally, from what I've seen on the forums here, the electrical system on the CTS is rather complex. Unless you have a lot of experience in car audio or possess some sort of engineering degree, I'd recommend finding a qualified car audio specialist (not the guy at the local Best Buy) to do your installation. This way you have someone else to blame if something gets f***** up. The consequences of an improper install can be wide-spread and complicated to diagnose.