Hi Guys,
I've had this same problem this afternoon after my 2013 MBA arrived.
Solution was quite simple in the end, I've listed my steps below for clarity but the main bit is the shut down I think.
1. Fire up bootcamp, already had win 8 key but selected "download latest drivers" after removing the WindowsSupport folder off my existing key.
2. Chose same win8 install key as target for drivers (created a new WindowsSupport directory)
3. Had last option for "install windows" checked too - partitioned drive and rebooted.
-- Reading through some of the posts it may be pertinent to mention my usb key was in the left port and is a Lacie iamakey --
4. Rebooted and hit windows installation, proceeded as normal
5. went through a few reboots with full keyboard and mouse support
6. arrived at personalise screen (windows setup) and had no keyboard or mouse support
7. hard powered off using the power key
8. booted in to mac os
9. did a complete shut down
10. booted back up in to windows partition
11. hey presto! mouse and keyboard support back.
I'm an IT professional so I should be able to hazard a guess at what the problem was there but sadly I can't quite figure it out. Certainly having the latest drivers on the USB key helped I believe (bootcamp installer popped up automagically when I eventually got in to desktop) and that last shut down/boot seemed to do the trick as warm restarting was getting me nowhere. Maybe the trip in to MacOS helped too - who knows!
Hope this helps.