How is this becoming public information so quickly? Is this being leaked? It seems like it's not beneficial for this to become public knowledge for anyone except Tumblr if they are trying to start a bidding war.
I suspect a big part of the reason that Yahoo! makes these acquisitions if for PR, i.e., "look! we're a dynamic company making huge moves!"
Look at Summly---how much value did they reap simply from the media coverage? They killed the software immediately, and I'm sure Yahoo! Research could've whipped up a pretty decent summarization engine at no extra cost.
