I'm pretty sure the Nvidia Kepler graphics line has CUDA, which enables hardware accelerated rendering adn such.
In which case, the iMac should fit all your amateur-based editing needs perfectly.
However, if you're a professional, I'm with most people here. The new iMac has its limits. the 21,5" model doesn't have user-accesible RAM, meaning that you're stuck with whatever you order it with. Also, the lack of an optical drive can, for some, be a killer. I've got libraries of stock footage and stock art on dvd's, which I won't be able to use if I go for an iMac, unless I buy the over-expensive super drive alongside.
I'm also kinda with Dorkman here, there just doesn't seem to be any viable reason to continue using macs until Apple gets their fingers out of whatever asshole they've been plugging them lately.
Adobe's available on both Windows and Mac, FCPX doesn't look like it'll be as good as FCP ever was, OS X 10.8 is too much of an iOS-like system, and each and every mac is now 'as tiny as possible', making huge compromises in hardware perfomance.
All that being said though, I'll come across as a two-timing, good for nothing piece of shit with this, but I recently sold my relatively high-end PC to switch to mac. Why on earth, after that rant would that make sense?
Well, I never liked windows much, with 7 being the best by far, and now 8 ruining the entire thing. I've also had hard drives crash on me several times this year, losing about 6000 photographs, and about 800GB worth of video files from shorts and other projects I've worked on. These crashes are also directly relatable to Windows, and while it's my own damn fault for not taking backups, I'm still sick of struggling with windows.
The only reason I bought the PC in the first place, was my tendence to play games every now and then, and the fact that I couldn't afford a Mac in that specific time period.
I'll be ordering a new iMac as soon as they're available. I'll even probably get a copy of FCPX. I'm not planning to go pro with my video work anyway, and I'm sure it'll fulfil my needs as far as editing goes. That and some Adobe apps.