Even if a company does not enter into a fixed-fee arrangement for all-you-can-eat from a law firm in a specialized area, it might look into an arrangement where short calls are not billed. Alternatively, short calls are only billed a set amount such as $100 for a call of less than 20 minutes. The basic idea, permuted into a myriad variations, encourages the law firm to be able to weigh in on questions but not have to create new matters or bill every dime (See my post of Dec. 2, 2009: Telstra retainer agreement).
Every area of law is a possibility for such a retainer. At the SuperConference, a speaker from Home Depot described such an arrangement for bankruptcy preference issues. Club Med created such an outlet for questions on Mexican law. A law department I know supports a retainer arrangement for environmental health and safety questions.
It is important with these deals to define who can call the law firm and where the line is that allows the firm to shift from the all-in covered to hourly billing. The arrangements have the benefit of predictability for both sides and they might encourage the firm to keep track of questions and store answers.