I love YubNub.
Besides just being very cool in a geeky way, it's got a lot in common with Skip. The interface that YubNub exposes to the user (a "command line for the Internet") and the Skip interface (a "click interface for getting things done with a phone") are diametrically opposed. YubNub is about typing out commands, while the Skip interface avoids typing at all costs. So it's not that part.
What Skip and YubNub have in common is the idea of adapting some of the great applications out there for a new context -- without rebuilding massive parts of those applications from the ground up.
YubNub rocks when your context involves keeping both hands on the keyboard. Maybe you're working in various console windows, or you're a writer or a reporter. I'd love to have this under a hotkey in Microsoft Word (maybe someone's already written the plugin). I type in a command like "gim porsche 911" and *bam* pictures of Porsche 911s. YubNub is extensible and has the notion of pipes and filters. So maybe I want to insert a picture of a 911 into my Word doc -- I could execute the search in such a way as to return just the first image, copy, paste, done. In the keyboard/console context, this is a powerful way to access anything that lives on the other end of a URL.
Skip does something similar, for the no-keyboard, no-typing context you're in while sitting at a stoplight or running from airport parking to catch a plane. We don't want to rebuild all of the airline checkin applications, limo reservation systems, or fast food point-of-sale systems in the world just to make them "mobile."
We'd rather build a context-specialized application that knows just enough about what you're trying to do to make it a whole lot easier, and lets companies get the transaction systems they've already built and operationalized literally into their customers' hands.