Most of you probably haven't the faintest clue who Zynamics is. They're based in Germany and used to be called Sabre Security. This is Halvar Flake's team; they sell BinDiff (the most popular binary diffing tool in reverse engineering, used by teams around the world to back security patches out to learn about flaws), along with BinNavi, a popular debugger-based reverse-engineering tool.
For the past few years, they've been working on an engine called VxClass that uses control flaw graph analysis to automatically classify malware. One presumes this is what motivates the acquisition. Google already scans content on the Internet for malware.
Or it could be mostly a talent thing. Google's been on a hiring tear. Extremely strong software security talent isn't easy to come by. Lord knows Halvar's got it. Zynamics is hugely well-known and respected in the software security world.