Robert Anbrogi has an excellent article posted at Findlaw:  Ethics: Hiring Experts You Don’t Plan To Use.   He asks:  "Do lawyers ever retain experts just to lock them out from being hired by the other side?"

"Lawyers do occasionally contact or ‘retain’ experts solely to disqualify them from working for the other side," says Erik Anderson, senior attorney in the corporate legal department of Safeco Insurance Company of America. He should know: he faced this situation in a case not long ago in which one party sought to disqualify the other’s expert.

Another lawyer who has seen it done is David W. White, a trial attorney in Boston who is also president of the Massachusetts Bar Association. Although he would never do it himself, he once found himself the victim of this tactic.

"It was an antiques case, where fraud was alleged," White says. "There wasn’t an available independent expert on the east coast of the U.S. because the plaintiff had consulted them all." 

When the expert is contacted, how much can be disclosed?  Typically the hiring attorney wants to be able to discuss that case with  the expert.  Must the expert refuse to hear any details before being hired?  Must you request a retainer just to talk about whether or not you want to take the case? Sure sounds like it.