student_trap makes some outstanding points. For me, I am (mostly) happily cutting away 1080i TV shows for air in my local television market (top 30). I have a 20" iMac (late 2006) and it does the job. The new iMacs are even bigger and faster and can crank out the work and I can't wait to get my hands on one.
But the bottom line is glossy screens suck. Other than adding more RAM, I can't upgrade the iMac. And the highest end iMac is "about" the same as the entry level Mac Pro. All these give the more expensive Mac Pro a longer useful life than the iMac giving owners a (much?) longer time to recoop and exceed their initial expense.
For me, I can't afford the added upfront expense or the decision would be clear. I love my iMac and I can't wait to get an i7. But, in reality, I really want a properly speced out Mac Pro.
Oh, and Nade...sorry, but that really was a stupid question. You set fire in a forum, no matter how cool we are, there's going to be flames. We're just having fun. Don't take it personal. We're just jealous 'cause the bulk of us don't have the student discount anymore.