Here is more information:

http://blogs.msdn.com/b/ieinternals/archive/2009/05/29/q-a-64-bit-internet-explorer.aspx

I'd discourage the useage of 64bit Internet Explorer.  According to Microsoft, 64bit Internet Explorer cannot be set as the default browser either:

http://answers.microsoft.com/en-us/ie/forum/ie9-windows_7/make-internet-explorer-64-bit-the-default-to-run/7f59d57a-14d9-4c95-b339-bc03631e4047

64bit Internet Explorer is known to be incompatible with a great number of sites and applications.  Plug-ins for Internet Explorer require special 64bit versions as well, and many of the common plug-ins do not (and may never) be supported on 64bit.

-dave