spidertools.com has an interesting little story with some great steps you should use whenever you engage a consultant.  The author using a forest fire analogy to make his points  about how you should protect yourself and your company from what I would call “Brain Drain”.  When you hire a consultant you should also make sure part of your contract includes documentation and training.  When I was consulting I always tried to get customers to understand and where possible actually be able to fix the problems they encountered.  While not every problem or solution should be attempted by a novice user of any OS there are plenty of situations where this is an option.  If you don’t put it in the contract you will never become familiar with any troubleshooting or solutions leaving you tied to the Consultant to update the server or fixing the problem in the future.