Here's a write up on the subject by MCN writer Kevin Ash:
Ride magazine did a test on helmets being dropped a few years ago and it was something I wrote about too - helmet manufacturers clearly have an interest in encouraging you to buy a new helmet if you drop one, and they also have to cover themselves as it can in theory be damaged in a drop, if it lands on a hard, sharp edge for example. But Ride lobbed helmets out of a first floor window onto a concrete floor then took them apart to assess the damage, and there was none in any of them. Nor should there be, a helmet is designed to take a hard impact while containing a 5kg head, and yes, I know it absorbs the higher forces by permanent collapse, but still when it's dropped while empty the forces are far less, there's much less inertia to deal with. Dropping it is also a very low speed impact for a helmet compared with what it's expected to deal with in a crash, and frankly even if it lands on a sharp edge from half a metre or so it's unlikely to sustain significant damage.
After I wrote about this in my MCN column, naturally there were manufacturers calling me irresponsible etc, but interestingly a few off the record, and Nolan on the record, said this was quite right, a helmet can withstand being dropped from typical bike tank and hand held heights without being damaged enough to need replacing.
