Now what this also might mean (anybody correct me if I'm wrong) is that by taking your computer from your works domain you loose this account's cached passwords and also you can't join the domain again because you cannot contact the domain controller (obviously).
That depends on domain policy. I think the default is 5 cached user/passwords. These are not reset by switching domains UNLESS you rename either a local account such as administrator or change the account password on the local machine while it is not connected a domain (windows gets confused). Oh there is a trick I learned, if you disconnect the cat 5 cable and try and log into a domain when it says "cannot find domain controller" it will use a cached domain password and let you in to teak with local setting (dependent upon permissions etc.)

For instance user1@bob.net is not the same account as user1@frank.net or user1@workgroup. Also when you don't have the domain drop down box available you are not connected to the domain but sometimes you can type user1@domain.com in the user box and sometimes get the right domain controller etc. Anytime you change domains you must reboot, change domain, reboot and join domain. That seems the only real foolproof way?