AT&T's site list the "approved" modems. 
EDIT
ATT Approved Modems
Which one is the best one?
AT&T Broadband - Approved Cable Modems 
   Manufacturer Model(s) 
3Com CMX2940, 3CR29220, 3CR29221, 3CR29223 
Arris/Nortel CM100, CM200, CM200U 
BestData CMX100, CMX110 
COM21 DP1010, DP101, DP111, DP1110, Doxport 1110 
D-Link DCM-100 
Ericsson PipeRider HM200C 
High Speed Surfing SM100 
Linksys BEFCMU10 
Motorola (formerly General Instruments) SB2100, SB3100, SB4100 
Thomson/RCA DCM105, DCM205, DCM215, DCM225, DCM226, DCM235, DCM235R, DCM245, DCM245R 
Toshiba DAZ8801, DAZ8811, DAZ8813, PCX1100, PCX1100U 
Zoom 5011 (external model only)