I'd say it's a purely semantic distinction, but the more I consider it, the more I see that it's not. At-wills have an exclusive status, being frequently modified by feats or, at times, enemy abilities. For instance, with the Dominate condition, an enemy can force you to use an at-will. The intent here is obviously, in part, that you cannot be forced to lost/expend limited resources (excepting actions) when Dominated; since Lay on Hands is technically ann at-will power, you can be made to exhaust its uses.
I feel it'd be a very dickish DM move, and certainly not one I'd use myself, but it's rules-legal.