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I discovered that going from the domain back to the workgroup, then back again to the domain is a mistake -- possible, but clumsy. I don't need the X200 at the office -- I use it at home and on the road, and I can still remote desktop to the office computers with it on the domain, so it doesn't matter. But I guess there is no easy way to use the same machine in my test domain, and then take it to the office to do work (except remote desktop, in a pinch).

Well not many people leave and rejoin domains daily lol. There's a reason you have to restart. Windows has to switch between group policy (from the server) and local policy everytime you switch around.
It's usually you're on a domain or on a workgroup.