There's actually a very good reason for my wanting to use a computer, a softphone, and the USB port. Phil Zimmerman has just released his ZPhone and I wish to use it in combination with an SIP softphone and an SIP provider.
ZPhone runs in the IP stack and establishes a peer to peer encryption session with the called party. The encryption is extremely robust, and more importantly, the encryption source code is available for public review. One can see for himself that there are no back doors in the ZPhone software.
Yes, some VOIP service providers use encryption, including Skype, but their implementation of the encryption is kept secret. Others cannot review their source code to look for back doors or alternate decryption keys.
Also, with ZPhone, since the end points of the VOIP call are performing the encryption, any attempt by agencies of the government to use the provider's equipment to spy on citizens' private conversations will fail, since the Zphone encryption does not happen in the provider's infrastructure.
Neil Salem, MA USA