I work for a US DoD contractor. For these jobs, they are based on "Information Assurance Training" levels. To admin systems, you need to be at least IAT 1 which means a baseline cert of A+ (or higher) and a computing environment (CE) certification. So, whatever the CE is determines the certification you need. I have MCTS: Windows 7 Configuration, Network+ and Security+. My company requires Security+ for the baseline certification. In addition to these, there is also the "obtain and maintain a secret security clearance".

If I wanted to admin servers, then I would need to get Server 2008. If I want to admin *Nix machines (which would only be servers), then I have to have the current Linux+ from CompTIA.

I'll be taking the CISSP test following a week long bootcamp and year long self study in June.