Yahoo must have really liked the download numbers for it's 2 existing Dashboard widgets. AFAIK, Yahoo didn't make Konfabulator versions.
In the short time Tiger has been out, Apple's site has over 800 widgets available now, pretty phenomenal growth. A lot of them are mostly eye-candy gateways for web-based services. For Yahoo, it's a good move for 2 reasons: it can "expose" widget-based technologies for it's numerous Yahoo services to the mostly widget-deprived MS Windows world in grand scale under it's own brand; and it gets to ditch the confusing, uninformative "Konfabulator" name. ("What's a Konfabulator???")
I tend to agree that Apple ripped off Konfabulator though. Regardless of the technical nuances, seeing is believing and when I first saw Dashboard, I saw Konfabulator. I think Apple had the right idea of making it an operational extension of the Expose layer. I already end up with too much crap on my desktop and my screen in general, and I haven't yet come across a widget yet that I saw the need for 100% full-time access to on my desktop. The idea of desktop widgets means that I likely have to close or hide app windows in order to get to some or all of them, then reopen my app windows again when I'm done just doesn't do it for me; hitting F12 for instant access to any and all widgets that I can put freely anywhere on the screen makes the widget concept much more usable.