Rumor is they'll make one. My gut tells me they will too. How can you pass that up? lol As for exceeding motherboard capabilities, remember the 7000 series uses PCIe 3.0. Of course if your motherboard is only PCIe 2.0, which most are, you may see a performance decrease (in gaming, not crunching), though it won't be very significant. It's kind of like plugging a SATA 3 hard drive into a SATA 2 port. No big deal.

As for the PCIe wattage specs, yes the 7990 would be over 300W, which is the "limit", but both AMD and Nvidia have both already crossed this limit in the past with the HD 6990 and Nvidia GTX 590, so i don't see that being something that hold them back either. I wouldn't be too shocked if a 7990 reference card popped up at CES in the next few days