I agree with a lot of what you wrote and it does apply for serious gamers, you will get more for your money with a special-purposed PC. However, I think you go too far with your point. I have the high end iMac (3.06 + ATI card) from this spring and it is quite a good and fast machine. I play Doom 3 and it is plays VERY well in Win 7 RC with Boot Camp, noticeably faster than my 2 year old HP with a good gaming card.
I agree completely, and my 2.93 + ATI 4850 does gaming very well too. But after years of frustrating Mac gaming and a few years of custom-built PC gaming, I've decided to jump to a console (PS3) instead (the $299 price helped). Macs will always lag behind PCs on the consumer hardware front, particularly with GPUs. Sad but true. Apple really needs to put the "premium" into its processors (including GPU) if it wants to maintain the premium pricing scheme. It baffles me that they're always 2 steps behind the Windows world with their video processors. And Mac gaming, software wise, seems to be going nowhere fast. I thought this would change with the recent popularity of Apple, but alas it appears not to be. (What the heck ever happened to Unreal Tournament 3 for Mac? It just vanished into thin air!)
And while yes, you can build a great gaming PC at a reasonable cost, you're still faced with a couple of major problems: 1) you're gaming against guys who are spending 3x (or much more) than you on their rigs, plus doing major overclocking and tweaking (i.e. gaming is their life). Good luck fragging that guy who is running a top-of-the-line dual (or quad) SLI setup with 16 gigs of RAM and an overclocked-out-the-wazoo processor that requires liquid nitrogen for cooling. And 2) you're gaming against a bunch of lamer punks who use hacks and dirty tricks, wrecking every game they join.
Console gaming seems to minimize these two problems. Equal hardware for all and difficult to hack.
So I'm officially going PS3 for gaming and using my iMac for everything else. Bummer there's no video in on the iMac so I can connect a PS3. Guess I'll be fighting the wife for the big TV.
Now I wish I hadn't sprung for the 4850...