Just found a nice link at holarse.de:

In short: It is a job offer for a senior software engineer that would be responsible for "Port Windows-based games to the Linux platform." and "Manage the operation of large clusters of machines running both Windows and Linux in a highly available system.". So is Valve finally gonna admire that there is a gaming market on Linux? Or are they only trying to get their server work natively on Linux, too?

Looks like time for some speculation has come...