Well, unlike Bearfeets, I can't say I'm all that surprised by the benchmark discrepancy.
I purchased the first generation 2.4 Aluminum iMac back when it was released last year and lived with it and it's ATI 2600 Pro card everyday until the revised iMacs came out. Like another poster in this thread, I picked up the new 3 ghz iMac not because I'm a gamer, but because I thought the machine would potentially have a better than expected life span given the specs of the machine generally and particularly, a video card with double the memory of the ATI 2600 Pro.
With Apple focusing more and more attention on the video card for graphics processing and overall pizazz delivery, i.e., quartz extreme, core image, core video, and now Leopard's much-hyped but so far vastly underutilized core animation (if you didn't know any better, you might think it was dropped altogether from Leopard at the last minute given the pizazz-ladened demos Apple showed off), the graphics card horsepower will likely become more of a factor in an Apple box than just frame rates in games - just as when Tiger hit the scene with it's new core graphics and core video technologies that was (and still is) largely dependent on cards with pixel programable engines. If you didn't have one, you were mostly out of luck, at least regarding those two.
So when the 3 ghz iMac was announced with it's new brawny Nvidia card, I picked it up with the intention of selling the 2.4 iMac to lessen the sticker shock. When I got it home though and set it up, I could notice a difference right away, and it wasn't good. Minor things overall, but still not a good sign. Specifically, I noticed the dock magnification effect wasn't as smooth as on the 2.4, with everything set identically on both machines. Oddly - or maybe predictably - setting the dock on either side instead of the bottom results in butter-smooth magnification. It's only noticeable (to me) when using the dock (with it's standard 3d shelf) on the bottom. In any event, the 2.4 with the ATI card has no trouble at all with magnification, be it a dock set on the bottom or either side.
Another thing I noticed is that entering (or exiting) the Dashboard is, more often than not, pretty choppy on the 3ghz machine. Seems most noticeable when invoking the Dashboard using the dock icon. Oddly (once again) if the keyboard or the Dashboard application icon in the applications folder is used to invoke the Dashboard, there are fewer instances of choppy entries and exits. Now matter what method is used, clicking the "x" to bring up the Dashboard bar seems ok, but when you close it, the lowering of the desktop effect is choppy pretty much 100% of the time. Again, my 2.4 iMac doesn't suffer a single one of these problems.
Finally, the last thing I noticed was scrolling in Safari using the scroll bar on longer pages produces a video tearing effect, which looks kind of like a transparent line appearing in roughly the middle of the page. Using the scroll pea on the mighty mouse doesn't seem to produce the effect as often, but the scroll bar is a cinch to reproduce it. Especially noticeable on frames inside pages with scroll bars. The 2.4 iMac? You guessed it. No such problem.
So, the very next day after I purchased it, I boxed up Mr. 3ghz, made an appointment with the genius bar at the Apple store, and bought it back. I reproduced everything I described here to the Apple genius, who agreed with me that there was a problem. He opined that since the machine was new member of the Apple lineup, it was likely there is going to be a software and/or firmware update forthcoming from Apple. But to cover all the bases, he also offered me a new replacement machine. I accepted the offer, but I also asked if we could check it out before I left the store. Sure enough, it had the same problems, exactly as described above. Two machines, fresh out of the box with the same problems
Nevertheless, I kept the replacement machine because these are relatively minor annoyances than real show-stoppers. Ultimately, Apple will likely get around to fixing issues with the Nvidia card, and one of the latest updates to the forthcoming 10.5.3 update mentions that new Nvidia graphics drivers were added to the update - after the 3ghz iMac was released. Maybe that might solve the problem with these machines. I hope so.
The bad thing is that I remember awhile ago there was a fairly significant problem audio problem that materialized in one of the last generation Powerbooks. It took months and bottomless threads of customer screaming on Apple's forums before it was finally resolved via a software update. Having an audio glitch on what was at the time Apple's premier laptop and taking that long to get it fixed was not a shining moment for Apple when you consider that audio production is one of Apple's most important, key markets.
By the way, iMovie 7 does depend on core video for some of it's effects rendering, but AFAIK, core video, like core image, are largely dependent on opengl for a majority of effects. Core video and graphics basically involve some fancy pre-packaging of opengl routines by Apple that are exposed to developers in a plugin architecture so they don't have to go through the hassle of coding opengl themselves in order to approximate a similar effect. So that opengl is apparently not an issue with games (which don't use OSX "core" technologies), but appears to be with some Apple OSX routines might open the possibility that the OS isn't making the right calls to the driver in certain instances - but that's pure speculation on my part.