Calling any landline was already possible with the gizmo project.

Remember, Skype is evil in many ways. Their protocol is very opaque, making it impossible to reverse engineer, which locks you to their software. Gizmo project probally does something similair with their own VoIP protocol, but at least they also offer a bridge to SIP, Yahoo, MSN and jabber.

I don't know how sound quality differs between gizmo and skype, but if gizmo doesn't do too bad, you should support their business model, and only use skype for skype2skyp communications.