When Microsoft's open source lab offered to help the Mozilla foundation with getting Firefox to run well on Vista, reactions ranged from skeptical and suspicious to surprised and excited. But in fact, the two teams have already been quietly working together. In this thread, Mike Schroepfer wrote "we are already in contact with your team via email". The lab time would be helpful, though, according to Mozilla developer Mike Beltzner, who added "yes, we'd definitely be interested in getting some 1:1 support".
It just goes to show that developers from organizations that are outwardly fiercely competitive are often quite civil with each other when nobody is looking. Perhaps that's why projects like Apache and Eclipse are so successful.
Personally, my first reaction was: "Microsoft has an open source lab?".