That entry has now changed:
http://www.microsoft.com/windows/compatibility/windows-7/en-us/Details.aspx?type=Software&p=Fallout%203&v=Bethesda%20Games&uid=&l=en&pf=0&pi=0&s=Fallout&os=64-bit
Microsoft now declares the game to be compatible with both the 64-bit and 32-bit versions of Win7. Perhaps they included a compatibility update in a recent round of patches?
At any rate, having just installed the retail DVD version of the GOTY edition on Win7 x64, I'm about to find out