I'm in the same position but in web development and already have a 17" Apple Studio Display.
Which...makes the iMac 24" even more desirable.

However, I don't need 24" right now and my current screen is fine for what I do.
I'm pretty much set on a base 2.0 Mac Pro. Wayyyy enough speed and I like to buy for upgrade ability. The graphics card being soldered to the board in the iMac puts a real stamp of 'finite life span' on it for a lot of people.
I lent towards the iMac 24 for the screen, the elegance, and the fact I was paying for what I'd be using. What I'm doing wont be running dual processors to any degree. I also read that even some of Apples own software doesn't utilize dual processors fully. The article mentioned exporting in Quicktime I think.
But I also need professional color accuracy. And several have said the iMac's screen was not made for that.
Also, what isn't a selling point for me, is the amount of internal stuff going on in the Mac Pro. I have no problem with external hd's. I have no problem with external optical drives.
So it's really back, forth, back, forth for me. An iMac
is plenty fast enough. Think it's gonna be a Mac Pro though. It also has the benefit of the unknown....if what you do (work, or whatever takes your fancy) changes, it will cover your ass to the nth degree. Ability to change screen later, USB 3.0, Firewire 1600

, graphics card if my needs change.....are all selling points. Plus, 60lb of aluminum is an investment if our country gets invaded and we're at war. I can throw it at the opposition. They will surely die.