No. And I'm not 100% certain that the snap-in adapter does either. This would be a function of the iPhone that isn't documented anywhere I can find. The adaptor for the iPhone is a standard iPod adaptor and I haven't seen any other accessories that boost an iPhone signal this way.

Ooops! I DID find one...

http://www.cellularaccessory.com/0-02-36-0015-2.html

Eject Box First, Then Snap in Adapter
 

X5WannaBe:

Yes, you have to have the Eject box which the snap in adapter mounts to. As said by others, the only advantage is the integrated look of the phone in the car, and the better reception gained by use of the external antenna.

I bought this iphone cradle 3 days after I had the car and I noticed an improvment to the signal as I have o2 here in the UK and it's not so hot but I notice 5 bars in the cradle and 2/3 bars just on top of the arm rest.
The cradle only charges the phone and uses the outside antenna everything else is via bluetooth - I use a seperate ipod with the little 'Y' cable for my music - hope i've answered your question