Andy Abramson got alerted to some interesting sections in Skype's help docs, which seem to suggest that Skype will be offering virtual numbers very soon:
Skype's Help section can be a plethora of information. A loyal reader in the UK sent along a note about SkypeIN. While I have yet to verify that In is Out, certainly the details are emerging. Do a little noodling around Skype's help and see what I mean. VoIP Watch: Skype In, Out?
I've set up a SIPPhone account, but haven't gotten around to a) configuring it on a free client or b) buying a virtual number. Actually, since I didn't buy any software or hardware from SIPPhone, there seems to be a bug: I can't buy a virtual number!
In any case, Skype is running on my desktop all the time. I would absolutely pay $5US/month for a virtual number plus voicemail.
It is no longer (or perhaps never?) about technology: it's marketing. Skype has an excellent, highly usably product with an excellent GUI sitting on desktops around the world. It's not getting any better with some people saying Apple should hook up with Skype. SIP needs a very large champion to overcome this advantage.