I was also just thinking of the same question today. All the USB phones that I have tried I never got to work properly.
I now use one of those USB to RG11 phone adapters like this one on ebay.
http://cgi.ebay.com/VOIP-USB-to-RJ11-Adapt...1QQcmdZViewItemwhich connect to your sound card port. I have only got this adapter to work with Skype version 1.3.0.53. The later versions
won't work for some reasdon . The Linux version I am using is PCLinuxOS 2007.
The phone will not ring if I get a Skypein call, but the computer speaker will ring. To make a Skypeout call, I still
have to call from the computer, but I can still roam around using the cordless phone.
Not exactly what I want.
I seems everthing out there is only Windows supported
I am looking at one of those DECT phones which require no PC