Is it possible to put a micro sim from an iPhone 4 into the Nexus S?
The Nexus S uses a traditional SIM card slot, but you should be able to do this provided that you get a proper micro-SIM to SIM adapter. Here's what I got from a quick Google search, and as you can see they are relatively inexpensive.

Also be aware that the Nexus S will not have 3G capability on the AT&amp;T network, as I explained in this question. This may or may not matter to you, of course.