I would definitely view the imac as a step down:
1. inability to use my 30" monitor (no I don't want 2 monitors on my desk)
2. inability to add internal drives (I already have 3 externals on my desk for backups. I don't want 6.
3. no graphics card upgrades (I know options for mac pro are limited, but at least there are a few).
4. inadequate cooling in laptop form factor which may impact component life in the long run (I like to keep my machines for 5 years)
5. inability to add a high speed solid state hard drive as my boot drive once price comes down and reliability improves.
6. very small number of connection ports (I have add-in cards on my mac pro for additional USB/FW that allow me to connect all of my peripherals).
7. mac pro has work station quality components and beefier backplane. Speed is not just about having latest/fastest CP, but mileage will vary depending on what you do with your machine.
I'm actually in the same position as the OP and trying to debate whether or not to get a Mac Pro or an iMac, as I also do music production and game. But I'll try to respond to your points to see if I can help give the iMac some leverage and maybe help clarify the comparison for the OP and myself.
1. Thats subjective. In a counter-subjective statement, as a music producer, I love working with two monitors, and the iMac would beautifully fill one half of that requirement. The objective argument here of course is that the Mac Pro can go well beyond 2 displays, unlike the iMac, where it is only limited to 2 displays, including itself.
2. Agreed. A strength of the Mac Pro is internal bay expansion.
3. Again, agreed. The Mac Pro is an obvious choice over the iMac in terms of gaming. But for light gaming, as the OP has noted, the iMac probably may be sufficient enough. I like to push the graphics on some of the newer games so I personally would definitely appreciate graphics upgradeability.
4. I would take "inadequate cooling" with a grain of salt. I have no technical data to back me up, but only the words of the community that I've read and heard. Apple obviously has a tremendous reputation as designers and their products have generally been to known to be long lasting compared to PCs. As for the latest iMacs, I've heard great things about how cool it maintains. If the general use of a computer is roughly 5 years, then I personally wouldn't worry about heating shortening your computer's life.
5. Same as #2.
6. For the most part, the Mac Pro simply ups one for each type of port, with the exception of two more firewire 800 ports. For an all-in-one computer, I feel like the iMac stands well on its own with connectivity options.
7. Totally true.
Ahh, decisions decisions. The 2010 Mac Pro needs an announcement already
