After configuring a Person/Group picker in InfoPath 2010 I couldn't get the "Filler" version of the form (client enabled, not web enabled) to successfully pull names from Active Directory. I thought that I had set everything up correctly, and I mostly had. In my answer I'm going to give a quick play-by-play for setting up a Person/Group picker.
First things first. Create an InfoPath form, put a Person/Group picker on it.
Now we set up the data connection in MS InfoPath
At this point my form was successfully able to access the AD listing. Prior to that I had gotten a "Server did not respond" error (or something along those lines).
I hope this proves to be helpful!