Just bumping this, am really wanting to know if anyone knows the definitive answer to this?
All i know is i downloaded the 10.6.4 update that was for the 2010 iMacs only, and noticed that it does have drivers for the Intel 82574L Ethernet Controller embedded in it. Odd considering that's the Ethernet controller used on the Mac Pro. I'd understand if it had drivers for other Ethernet chips too, but it doesn't. The normal 10.6.4 update has that same set of drivers, but also has others too. So i can't help but think it's deliberate that the drivers for only that controller were included in the iMac 2010 only update. Might be wrong though. That controller of course does support Jumbo Frames.
Also, people have noted that the tech specs on the Apple website don't mention that the ethernet port supports jumbo frames on the new iMacs, yet the tech specs on the Mac Mini and the Mac Pro do. Oddly enough the tech spec pages for all the iMacs going back to 2006 neglect to mention the fact that the ethernet port supports Jumbo Frames, yet they did.
Any new iMac owners fancy going to the Apple System Profiler and telling us what Ethernet card is in the new model ? 😀