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On our Thinkpad X60, the modem was connected to COM3. In tests, the virtual machine was able to easily issue commands to the Thinkpad's modem and connect to one of iPass' local poins of presence (for dial-up access to the Internet). One thing we noticed though is that you have to be careful about trying to map a VM's COM port to a physical COM port when that physical COM port is already "mapped" by another VM that's in use.
For David Berlind's write-up on The Virtual PC 2007 User Interface, see his post in ZDNet's TestBed blog.