BMW have not yet released the iPhone 4 snap-in adapter and it may be months before they do that.
I have little patience so I modified my iPhone 3G snap-in adapter so that it would work with the iPhone 4. After some work with a Dremel, I was able to make it fit smoothly -- it also clips in like in the 3G does. It charges successfully. I've not yet noticed any antenna improvement -- I had read somewhere that one will not be able get antenna improvement on the iPhone 4 because of the new antenna design used in that phone.
My wife's iPhone 3G still works just fine in this adapter.
Here's two "after" pics. The first is just to show the "charging" icon in top right.