iPhone 4 'direct control'

TraderDJ

I'm looking at radios that allow control of the iPhone (4) via USB with steering wheel controls.

I know they need to say they are compatible with steering wheel controls, and with iPhone control. I've narrowed it down to a few - such as http://www.ebay.co.uk/itm/PIONEER-DEH-2400UB-CD-MP3-WMA-Car-Stereo-USB-Aux-In-Player-RDS-Tuner-iPod-iPhone-/250958330009#ht_2499wt_1344

My question is when plugged into the USB, will the iPhone still work (to use TomTom/Sat Nav apps, not texting etc!) or will it 'lock down' and just say 'connected' as the ipod picture on the Apple website shows (http://www.apple.com/ipod/car-integration/images/ipod_car_recognition_20080508.png)

Basically I want to plug the iPhone in, load TomTom and have that giving directions etc, but also be able to change songs from the steering wheel, without affecting the maps etc. Will that work?

Thanks