The first step is an initial discussion with who'd be your boss and a technical quiz. The quiz is fairly easy, and consists of questions like What's the default port for FTP (21), what are the Active Directory roles and so on. This isn't meant to be open book, it's meant to see what you know off the top of your head. Time won't be given for Google.
The second step is a break-fix. You will log into a Windows 2008 R2 server and perform a series of tasks. Some include making modifications to MSSQL, IIS, and Windows NTFS permissions or group policies. It's time limited but it's open book so you can Google all you want.
Finally it's a panel interview that will last about three hours. You will interview with no only your future manager, but other members on the team. They'll test your knowledge of Rackspace, why you want the job, situational scenarios, and eagerness.
The best advice is just be yourself. Don't try to BS your way through. If they sense any insincerity at all you won't pass.