If I have AT&T, will the iPhone 4S work in areas of CDMA coverage only?
No. CDMA will only work of you buy and activate your 4S device onto a CDMA carrier. 

The FAQ shows that when you activate the phone it will choose one technology for coverage in country. GSM Registered phones will roam on GSM as will CDMA phones. Sprint should work in 35 unnamed countries and Verizon in 40 at launch.