Pioneer's site says the adapter box is compatible with DEH-P880PRS... is DEH-P800PRS the model you actually have? If so, you might want to verify with Pioneer, as certain model numbers may not be compatible.
I'm not sure about just the IP-BUS cable working either. In theory, just having the cable would allow all iPod data to show on the HU, but you have to control song play through the iPod.
The adapter is so you can control of your iPod via the HU or iPod, and all data shows up as well.
I'm not 100% on this... can anyone else verify? I have the CD-IB100II, and I love it.