You could try actually looking at the blackberry website before buying models, it will tell you if its GSM (sim slot) or CDMA (no sim slot) or a CDMA with sim slot for international use.
AT&T is GSM, and therefore has SIM slots, and uses IMEI.
ESN is typically used by CDMA carriers.
A "bad ESN" means one that is blocked and cannot be registered with a carrier.
"That's it then. Cancel the kitchen scraps for lepers and orphans, no more merciful beheadings... and call off Christmas."