Rainer Brockerhoff has an excellent analysis on Apple’s TPM chip usage and possible future plans for OS X 10.5 (a.k.a. Leopard).
So, what I think likely is that the machine will boot into the trusted hypervisor. This will be encrypted into firmware and decrypted, and checked against tampering, by the TPM chip. Once this is running it will show a screen like the Boot Camp boot selector, with one important difference: you’ll be able to select more than one OS to boot up. All of them, including Mac OS X itself, will run inside a virtual machine.