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)