From Apples Knowledge Base
iTunes 7.7 and later has the ability to update the carrier specific settings on iPhone with iPhone 2.0 software or later. Carrier settings updates are small files (about 10k) that are downloaded from iTunes to be installed on your iPhone. The carrier settings can include updates to the default APN (how iPhone accesses the carrier's cellular data network), special dialing codes at Settings > Phone > [Carrier] Services, default settings for Stocks, Maps, Weather, and other items.
So what is in the update? After extracting the .ipcc file, we saw two new images. Both for AT&Ts upcoming 3G MicroCell we wrote about here. The images will most likely be used to indicate if you are on the 3G MicroCell "Network."
(The other image is a grayed out version of this)
AT&Ts 3G MicroCell might be coming sooner than we think.