They are more than one might think. Virtualization products like VMware, VPN clients like Cisco's, system optimization tools, hardware monitors (temperature etc.), Finder, Expose, and Spaces tweaks. I always wonder who would use the latter, but there some to be considerable shares. Just letting them do an upgrade and setting their default kernels to 64 bit could create a huge mess that would not be worth the tiny improvements in the light of public perception. Rolling out 32 bit first is a sane decision.