I've seen this problem rear its ugly head in a number of instances that cause various, interesting problems. For example, there are two major Audio Video brands: Allen & Heath and K & M Stands that both feature ampersands in their branding. The problem we've had with it is from a database perspective. Ampersands just cause all kinds of problems. Still, these brands have been around a long time.
Think "M&Ms" - no one really searches "M and M" [unless it's "Eminem" ] - So, I think it's a matter of choosing one for branding purposes and sticking to it. In your case, with a law firm, searchers are really searching proper names (I'm thinking). So if I were to search for a string of last names, I might be more likely to use "and" instead of "&" - Until the "&" becomes part of the branding, I think Google is probably going to defer to the "and" version. Even if the law firm was huge, I'm not sure it would ever reach that level of brand recognition. Some successful brands with ampersands [Just for fun!]:
H&M, A&W Rootbeer, A&P Grocers, Proctor & Gamble, B&H Photo, Bang & Olufson, AT&T, Ben&Jerry's, Arm&Hammer, Boys&Girls Clubs of America, A&M Records, Bed, Bath & Beyond, Johnson & Johnson, H&R Block, Ernst&Young, Fod&Wine, Black&Decker
(Thanks to Sporcle, but they left M&Ms off their list! How can that be?) Thanks for the fun question