Skype is refusing to certify webcams that do not have their own CPU's built in for it's Skype HD video calling which offers 720p resolution. That doesn't mean non-certified webcams won't work, just that Skype isn't saying by any means that they will.
One example is the Creative Live! inPerson HD which handily has it's own CPU built in allowing it to process the images on a laptop that normally wouldn't have been able to cope. It's a clever idea but does come at a premium, the inPerson HD costs a cool £109.99.
Of course once you have a HD webcam all of this is then reliant on how good your internet connection is. In essence this is Skype covering all the bases making sure their HD service is exactly and only what they're promising.