According to a WebmasterWorld post, Yahoo! is stealing clients from its search marketing consultants. Google has been accused of the same before. The thread creator, who is already a a Yahoo! SEM, had this to say-
However, two of my clients, who already had sponsored search accounts, were called [by Yahoo!]. One of them actually decided to cancel with us, believing the free fast track service was all they needed to manage their account, and our expertise was no longer necessary. The other client referred the cold caller to me, saying we handled ppc for them. This guy had the nerve to ask me to refer any new customers we have to HIM.
The truth, however, remains confirmed. To the best of my knowledge, both Yahoo! and Google check with every new client whether he is already an existing SEM's client or not.