This is cool interview with Michael Robertson, of MP3.com and Lindows fame, who's now pushing VoIP based on SIP with his new SIPPhone. The article points out the differences in strategy - Skype's standard is proprietary but isn't tied to hardware, whereas SIPPhone is using SIP, which is more widely used, but is tied to phones.
Also interesting to note the brewing platform war issues - Robertson insists he won't support Skype. and that they should interface to him.
An alert commenter also points out that Skype has the technological edge right now, since they've solved firewall, bandwidth and encryption issues the SIP guys haven't.