Microsoft shared some interesting information about the 64-bit versions of Windows 7.
"As of June 2010, we see that 46% of all PCs worldwide running 64–bit Windows 7. Compared to Vista at 3 and a half years after launch, only 11% PCs running 64–bit Vista worldwide. The reason for the jump in transition to 64-bit PCs can be attributed to a few things. The first is the price of memory has dropped over last several years making it easier for OEMs to up the amount of memory in PCs they ship. And most major processors in PCs today are capable of running a 64-bit OS. There're also more and more compatible devices and apps for PCs running 64-bit Windows 7.
OEMs today have fully embraced 64-bit. We've seen many OEMs convert entire consumer lines of PCs to 64-bit only – which can be seen quite a bit today in North America. According to Stephen Baker at NPD, 77% of PCs sold at retail in April 2010 in U.S. had a 64-bit edition of Windows 7 pre-installed."