To expand on that just a bit, the computer will only have an account in one
domain at a time. If you want to move a computer between domains, you must
first disjoin one then join another. This will affect how domain computer
polices are applied.
If you have the horsepower and RAM on a PC, VMWare workstation (check out
the new VMWare Player product) can enable you to run multiple VMs joined to
different domains at once, all accessing local PC resources.