[wolrah]

The Athlon 64 was released in September 2003, the Prescott Pentium 4 in February 2004, and the iMac G5 in August 2004.

Desktop CPUs have been basically guaranteed to be 64 bit since 2005.

The Pentium 3 derived Pentium M and Core Solo/Duo delayed wide mobile adoption of 64 bit until Core 2 came around in July 2006. I'd guess by 2007 you could probably safely assume 64 bit across all but the cheapest laptops as well.

tl;dr: Aside from a few Atom-powered netbooks 64 bit has been standard in anything resembling a normal computer for around a decade.