Will time machine wipe out my old time machine backups if I move it to a new mac (or upgrade os x)? I recently had the IT guys here at work wipe my hard drive and upgrade my mac to 10.8 from 10.6. And other than spending a few days getting it up and running again and spaces being jacked it is going ok.

I was using Time Machine to keep it backed up for a long time and I want to use it again on the same external drive. But I am concerned that my most recent backup from my old backups will be lost if I set it up using the same drive. Will Time Machine wipe all my old data or just the oldest stuff?

Thanks!
No.
Time Machine will just start backing up your Mac along side your old Backups. Your previous machine will be back on Tuesday (or whenever last backed-up) in your timeline, with the new machine after that.