The 32bit claim is false. I eventually got 5 GPUs working on 32-bit 11.04 Ubuntu Linux, without any hacking of the ATI drivers or APP SDK.
That's correct, didn't catch that before. Both the 32bit and 64bit drivers should support at least 8 GPUs currently.
The driver's interface with the OS layer may use 32bit or 64bit addressing depending on the OS, but the binary bits of the driver are likely identical otherwise. The newer Radeon HDs actually use a 256bit address space to address it's own memory, internally - the exchange with the OS is all serial communication (PCI is serial), so addressing the GPU components from the OS level is just about sending the right hardware address + commands.
It could easily be a 16bit OS and address an infinite number of GPUs, as long as the driver handles interfacing that with the OS properly.