64-bit build didn't work (couldn't find ws2_32.lib) until I ran the SDK Configuration utility and selected v6.0A as the default SDK. This may be because I forgot to explicitly select the 64-bit compilers when I initially installed VS 2008, but it's a worthwhile troubleshooting approach.

Still building some of the big projects now, and I have to leave it again, but it looks like everything is okay once you get past setup. 

I'm +0 on this change now, and I really hope the buildbots will pick up any subtle changes in the build. We should also run as many tests as possible against existing extension modules.