QUESTION :
Will a newly purchased iMac come with a 32-bit or 64-bit operating system?
ANSWER :
Snow leopard runs 64 bit on Intel CPUs (PPC processors are no longer supporte), so a new iMac will run the operating system in 64bit mode. Applications will run with 64bit too if there is a 64 bit version, otherwise of course 32 bit software runs fine too.
The kernel will still be 32-bit unless you tweak your configuration.
Rest of the system (applications, services, …) is fully 64-bit (except iTunes for the moment).