I don't overly buy the 'pent up demand' argument either. Although initial sales would have been for that, a lot of Camaro enthusiast dismiss this car as not being a 'real f-body'. They prefer the older, lighter car that easily takes engine swaps and rear end swaps.
Mustang buyers would have bought Mustangs in those 8 years and Camaro buyers would have kept their 4th gen Camaros or bought something else. Both would be in the same position lusting after the new versions of their favourite pony cars in 2011. If anything, I think retention is just as strong of an argument. A Mustang guy who had a 98 Stang, then a 2005 Stang is probably ready for a 2011 Stang. A Camaro owner who had a 98 Camaro, but then was forced into something else when it went out of production in the mid-2000's (Impala, CTS, Charger, etc?) may like the trade-offs in their new ride better and is not inclined to get a new Camaro/Coupe/Pony Car.
Another factor I believe in its strong selling is how much the V6 looks like the V8. We all know at least 2/3 of these cars are V6's. With the car looking the exact same as the faster/fancier version, it may entice more sales; even the rims are the same. The Mustang in 2011 has finally done this as well with V6 looking a lot like the GT for the first time in a long, long time. Take a look at how much more pedestrian the SXT Challenger looks then the R/T or SRT-8.