64 bit will never be more compatible with the current 16bit and old er problem 32bit apps. but the same 16 bit apps will be replaced by newer 32 bit apps. This will take longest in the corprate market where ther are still system running DOS, Windows 95/98, NT4 and other old systems.
The people who write the software for the banks are very carful and write to the O/S in use NT4/2K are common and the banks are even more carful and will not upgrade the O/S untill ALL of the apps are tested, supported and fully validated on the newer O/S.
With this in mind I would say that the corprate space will be at least 7-10 years before it moves to 64bit O/S but the home and small-medium business (less than 500 computers) will be over 90% 64bit in...