As Raymond mentioned in his post, the MSDN site finally has some decent documentation on the x64 calling convention, including:
Alignment Requirements for x64 64-Bit
Register Usage for x64 64-Bit
Calling Convention Process for x64 64-Bit
Stack Usage for x64 64-Bit
Prolog and Epilog Code for x64 64-Bit
Exception Handling for x64 64-Bit
Other Issues for the 64-Bit x64 Calling Convention
