Have you upgraded or reloaded your OS? As Crucial pointed out, there is an OS mod you have to make to use BBMaps on AT&T. If you've changed your OS, you may have reactivated that block. Here're the steps you need to follow to enable BBMaps on AT&T:
Follow this path
C:\Program Files\Common Files\Research In Motion\Shared\Loader Files\8310-v4.2.2.176_P2.5.0.34
Find the LBS.alx (make a copy of it and leave it there)
Open it up the orginal
Type CTRL+F and search for this net_rim_bb_lbs_ribbon.cod
You should see this right above what you just searched for.
<fileset SystemSize="normal" Colour="True" Java="1.0" _vendorID="~
102|124|137|119|162|142|183|147|217|175" series="~8800">
Just delete the 102| highlighted in red above.
Save the file.
Connect your BB and the Launch the Application Loader and make sure BBMaps is checked and load it up. If that doesn't work just use the OTA download on the
www.mobile.blackberry.com site to install it.