I've come up with a workaround that seems to do the trick under 64-bit Windows 8 and ISE 14.4.
1) Install 14.4 like normal
2) Use the 32-bit version of Project Navigator to do your editing and compiling
3) Launch the 32-bit version of Impact (Xilinx\14.4\ISE_DS\ISE\bin\nt\impact.exe)
4) Launch the 64-bit version of the Remote Cable Server (Xilinx\14.4\ISE_DS\ISE\bin\nt64\cse_server.exe)
5) Configure Impact to connect to a remote cable server at localhost (explained below)
6) Initialize your chain, assign a configuration, and program as normal from that point on
The problem we're facing under Windows 8 64-bit is with the open file dialog. The platform cable driver itself works fine. Since you can't open a configuration file in 64-bit Impact without it crashing, it's necessary to use the 32-bit version. The 32-bit version does not have this open file dialog glitch. Since the 32-bit version can't interface with the 64-bit platform cable, it's necessary to use the remote cable server instead. This steps around the issue.
If you haven't used the remote cable server before, here is what you need to do: You can launch the cable server by running cse_server.exe in your nt64 folder (Xilinx\14.4\ISE_DS\ISE\bin\nt64\cse_server.exe). At that point, go ahead and plug in your platform cable and let any drivers install if you're using a USB platform cable. I don't think the parallel version will show anything on screen. At that point, run the 32-bit version of Impact. On a 64-bit computer no shortcut will have been made, but you can simply run it from your nt folder (Xilinx\14.4\ISE_DS\ISE\bin\nt\Impact.exe). Double click on boundary scan like normal. At that point you will have new menu options under the Output menu; select "Cable setup". Select the type of cable you're using in the Communication Mode section and then select the "Remote" cable location below. Type localhost into the "Host Name" box and click OK. At this point, you should be able to use Impact like normal.
