They have been giving each other thumbs up or thumbs down. Then, in a rewrite of history, Digg was invented and if you believe the sites rabid fans figured out that you could give content thumbs up or down for content rating. Oh, I forget, they added a gradient box with a bevel that surrounds the running tally.
I love Digg. It's totally replaced Slashdot for me. You hear about news quicker and you get a sense of how important a subset of the population thinks each topic is. So Yahoo goes and applies collaborative content filtering to a public suggestion database for their products and gets ripped up for "copying Digg". Apparently Digg even comes with it's own Slashdotter-esque crazy religious following. One less reason to go to Slashdot!
Ignore that "imitation is the highest form of flattery" line for a second and think about it. Yahoo didn't pull an AOL and use thumbs up/down for news filtering ala Netscape.com. They used it as a way to figure out what their customers want most... a wholly unique idea. I'm willing to bet that this wouldn't have even been noticed (because it's really a non-story) if they had simply not used the "digg style" gradient box flat list UI... which a lawyer could probably argue is reasonably obvious anyway... it might not make it right to copy the design so blatantly a good idea, but it's a feed... a flat list... of news items... with a content rating... something just about every blog engine supports.
The Digg users that are angry should forget about the specific UI for a second and realize that what makes digg powerful is that they build a userbase that no other online site <cough> Netscape </cough> has been able to replicate. The fact that there are so many thumbs voting is much more powerful than letting people vote with their thumbs and whatever silly gradient boxes you put around the rating. And, again, Yahoo isn't using this to filter news; they are using it to find out what their users want to improve in their products.
Move along sir... nothing to see here.