I don't get it. No matter how you spin it Yammer is just a Twitter for businesses. Companies like GoodGuide and Swype were much more deserving of the grand prize.
Google was yet another search engine. iPod was yet another MP3 player. Doing something better (or for the right audience) is often what it takes.
That being said.... I dunno if Yammer captures the motivation for USERS to "yam" (tweet?). I can see why people use twitter. I can see why businesses would want Yammer. But I can't see why business employees would want to use Yammer.
Because not Yammer-ing enough will look bad at your performance review. Ah, I love the corporate world.
I do think there were other "cooler" companies, but this could be huge. Right now Twitter mostly consists of a relatively small group of web/tech people. Just like what happened with Facebook and colleges, if Yammer gets critical mass in a few big companies, it could explode.
When Twitter launched people said it was "just" a place for people to post what they're eating and that it wasn't nearly as worthy as feature rich services that had come first (Upoc and Dodgeball).