Apple has pushed a carrier update and it added support for AT&T's micro-cell.

Here is the new strings in the carrier.plist:
<dict>
<key>AllowPrefixMatching</key>
<false/>
<key>CarrierName</key>
<string>AT&amp;T MicroCell</string>
<key>DefaultImage</key>
<string>Default_CARRIER_ATT M-Cell.png</string>
<key>FullScreenOpaqueImage</key>
<string>FSO_CARRIER_ATT M-Cell.png</string>
</dict>