nVidia realized AMD really didn't have much to compete with Intel this round, so they made the 680a as a tribute to AMD's QuadFX platform. Any other AMD-socket nVidia boards are low-end material. Hell, they don't even have a 650a.
AMD 790FX is set to be a phenomenal chipset, no pun intended. nVidia's nForce 790 is sounding a lot less impressive, if you ask me. It's just a 680 with a PCIe 2.0 bridge chip providing 32 PCIe 2.0 lanes. As to a new nVidia MCP (south bridge), I believe I've heard they're updating it. I don't think they're updating the MCP in time for the release of the 790 though. I could be wrong.
It just seems to me that if you want connectivity and options, you go with an Intel X38. If you want SLI, you go 680i. AMD is a non-option in the high-end market currently. It also sounds like both the Phenom AND the HD3000 series will be disappointments. So, as tempting as the 790FX chipset sounds, I personally wouldn't go for it unless the primary focus of my rig was not gaming. It looks to be low-power, high-connectivity. And add the ATI SB700 to the mix and it'll be a killer motherboard solution.
Too bad AMD can't match their chipset with a worthy CPU and GPU yet. I'm really hoping they'll put something out that astounds, a whole platform that's worth every piece, instead of a great motherboard and a so-so GPU and CPU.
Also, 790FX will be at the heart of a QuadFX system, which will now include CrossfireX instead of SLI. However, 790FX is definitely a single-socket enthusiast platform as well.
Again, too bad they can't make a CPU and a GPU which would truly classify it as a high-end enthusiast system.