The 2012 iMac 27 is technically current, even if it hasn't been released and is the high end iMac, so I'll stand corrected there.
The iMac 21 comes with a 640m, which from I've read is is average at best for a decrete GPU in 2012 but the next generation will probably be announce and might be shipping before the Apple has the both the 21 and 27 iMacs freely available.
I shouldn't confuse the hardware that you should get vs what you do get and whats current. I will stand by the statement though that Apple does not always keep up with current GPU, CPU and IO. USB 3.0 started shipping in 2010, Apple shipped it in 2012. The Mac Pro still only has 2.0.
As I think about it... Apple GPUs are usually current (except MP), they are just usually horrible for a company that produces expensive workstations and claims to love visuals. A $1,700 rMBP 13 with it dual core i5 and HD 4000 driving two 2650x1440 displays under load very well.
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I will concede that the hardware is USUALLY current, even is not a good value or choice. However it is not always current, that hasn't been true for years.
I'm willing to suspect that the USB 3 adoption was slow because they wanted to push Thunderbolt. I have a 2011 Lenovo ThinkPad workstation laptop that has USB 3.0 but prior to 2011 no manufacturers had added it yet. Apart from the Mac Pro which in my opinion needs to be sent the way of the Dodo, all models have the current generation CPU and GPU but they are not the highest-end available.