It is important to note that not all core2duos are mobile processors, and not all i5/i7s are desktop processors. Intel makes too many processors as it is, and they all share same names, which is very confusing.
I'm pretty sure the core2duos in these latest generation of iMacs are wolfdales, so they're desktop variants of the core2duo cpu. They offer very high clock speed and great power for their amount of heat. In most cases you won't notice any difference between a high end core2duo and a low end lynnfield like the i5 750, which is the baseline cpu in the nehalem iMacs. The difference between the two is 2 cores to 4 cores and an advancement in cpu architecture that mainly provides turboboost and better cpu performance.
Since you're gonna be using adobe products like premiere pro, I would definitely recommend the lynnfield i7 860 variant of the iMac, because of hyperthreading-- something very useful in video editting/encoding. Even if you weren't, choosing one of the nehalems would be the smart move, because you're buying an iMac with the mindset that you'll be keeping it for 5 years. 5 years down the line I'm sure more things will be optimized for nehalems than core2duos.