The problem is this doesn't work for some of us, and I suspected it was something to do with my Ultrabook only having USB 3.0 ports, since my other Win8 x64 box with USB 2.0 ports has no such issue.
I had a poke around and found this site here which shows you how to remove the Microsoft USB 3.0 drivers and instead install the Intel USB 3.0 drivers on Windows 8.
After following the procedure carefully, I can confirm this fixes the problem for me on Windows 8! After successfully installing the drivers, fastboot started working immediately for me.
Step 1: Install the HTC drivers from the standard HTC Sync Manager (Win7/8). If you are on Windows 8.1, please use the HTC Drivers linked later in this thread here (thanks Kardus!)
Step 2: Hit up Plugable's site which has a great step by step explanation of how to install the Intel USB 3.0 drivers. Full credit to Jerome for writing the guide, and Ekko who did the original work.
UPDATE: As per Coolookoo's discovery (Thanks Coolookoo!), if you are using a 4th Gen Haswell based Intel CPU, then you need to get the Intel USB 3.0 drivers from this link here instead of the link at Plugable's site. You will however still need his modded .inf files.
Step 3..... Profit
You should now have fastboot working flawlessly on Win8 x64 with USB 3.0, and as a added gift some older 'incompatible' USB devices that you may own should also now work under Windows 8.
Would love to hear some feedback on how this went for you.
If it helped, please hit the Thanks button!