This would be a first I have ever heard of this. I honestly don't know how something like you describe could ever happen. While I would never say it is impossible, I do think it is highly unlikely for the BB to swap phone numbers like this.
First off, is there anything at all common about the data swap? Are all the contacts and addresses correct, except for the name--on each record.
If you have correct the list, do so on your PC, on the Yahoo site where it is easier to correct rather than on the BB. And then sync again to update the BB.
Still, very very strange.