I finally have a new Dell Precision T5600 Windows 7 PC on the way!
It has 2 USB 3.0 inputs.
I have a Phonic Firewire board that never works right even with the recommended TI chipset. I refuse to go through that aggravation again with the new PC!
 
Are there any mixers/multi-channel audio interfaces available for USB 3.0 that support recording of 8 channels simultaneously?
 
 

Apparently USB 3 will make zero difference to audio streaming so is not on the radar for interfaces ( yet) USB 2.0 has  more than enough bandwidth for 8 tracks. I asked this same question about 2 years ago and I was told to the effect that USB 3.0 has faster transfer speeds which is not the issue with audio streaming. It's the bandwidth that counts, not speed. So therefore no benefit to us audio people.  

It has 2 USB 3.0 inputs. I have a Phonic Firewire board that never works right even with the recommended TI chipset. I refuse to go through that aggravation again with the new PC!  

There are currently no USB-3 Audio Interfaces... and (to my knowledge) none that have been announced.
Note that unless you have a Z77 or Z75 chipset motherboard (where the USB-3 controller is integrated into the chipset), you'll likely achieve better performance connecting the USB-2 audio interface into a USB-2 port.