deadsea - 7:26 pm on Feb 14, 2011 (gmt 0)
Let me refer you to a similar problem that somebody posted about last week:
I have experience with two such mistakes and SEOMoz did an experiment on a blog that did the exact same thing have done.
In all cases, mistakes like this can be disastrous. Expect to recover half the traffic that you have lost, but the rest is going to take months to come back.