After several weeks of fun getting all of calibre's dependencies to build on Windows 64bit, I finally have a working 64 bit build of calibre in Windows.
The installer is designed to install calibre 64bit alongside an existing calibre 32bit install. So you can switch between them until you figure out which one you want to use. The shortcut for launching calibre 64bit is named "calibre 64bit - E-book management" and when running it the status line at the bottom of the main calibre window will say [64bit].
The advantages of the 64 bit build are it can use all the RAM on machines with more than 3GB of RAM, which allows conversion of certain large and messy documents. There also may be small performance improvements.
The downside is that since this a new build there may well be some bugs that lead to crashes.
So if you are running a 64 bit version of Windows, try out the new calibre 64bit build and let me know how it goes.
Note that calibre has had 64 bit builds for linux and OS X for years.