Lee,
Do you have something more to go on? I'd like to follow up on. Using captured artillery wasn't all that uncommon for the western allies, but I'm unable to find reference anywhere captured tanks were used. (Motorcycles, trucks, even half-tracks, but not tanks.) In a way, it wasn't worth it. The western allies were never short of equipment and vehicles could be replaced within days or a week at most.
Unlike the Russians, who trained crews and support personnel on German equipment and organised a logistical chain, Americans/Commonwealth/etc. crews climbing into a Panther would be confronted by a completely unfamiliar and overly complicated machine with no spares and little chance of repair or resupply. Tactically it would seem like only in the most dire situation would a crew bother to man a captured German tank, but not out of the question.
There's also the danger of attack from the "American Luftwaffe". That is, the USAAF. Fratricide from ground-attack aircraft against marked Shermans were common enough, I can just imagine how a captured tank would fare!
At any rate, doing some research I see that that the British did use captured Italian tanks in North Africa and the Australians used both German and Italian tanks, but can't find any references in Europe after Normandy of the western allies or specifically Americans using captured German tanks.
Unrelated but fascinating: the "German Tank Problem". How statistical analysis of the serial numbers on captured tanks allowed for really accurate estimates of German tank production in WWII.
http://en.wikipedia.org/wiki/German_tank_problem
Tony