I have never seen any number of apple units in service. RIM is the only one that has numbers on that and thats because they route everything through the central data center. Even then their are units out there in use without a data plan so thats not 100% correct.
Apple and others have no way to track the numbers of devices in service. It would be nice to know but its not going to happen.
"A computer lets you make more mistakes faster than any invention in human history - with the possible exceptions of handguns and tequila." - unknown