All Pioneer decks manufactured in the last few years run on custom builds of WinCE; by going through a reset button sequence during the boot process you can gain access to your WinCE desktop, complete with taskbar, start menu button, and registry.
What you do from this point depends on your desired goals; there is an open-source project called AppRadio plus, which is available on Avic411, or you can write your own custom extensions for WinCE, which is what I have done. When I first played with the AppRadio at CES I loved it, since it was a development model and had no nag screens. In Europe, they have a few nag screens, but in North America, you're presented with a Nag Screen perhaps once every 5 minutes - it's highly annoying. Also, Pioneer equipped their AppRadio with a difficult anti-bypass system, which requires a signal pulse, rather than a ground connection, which makes it very difficult to bypass reliably.
With the confirmation and nag screen dialogs stripped from the deck, the experience is quite good. Of course, Pioneer has shown their concern for liability, and as a result, it's unlikely they will create a deck with the necessary interface for high levels of sales. It's unfortunate, since this is a concept which there is a strong market for.
