I found an ASUG document titled "mySAP™ ERP 2004-2005 UPGRADES: LESSONS LEARNED FROM SAP CUSTOMERS." It was written by Hercules Bothma and Vanessa Smith. It contained great stats about the number of customers on each version of R/3 and drivers for upgrading.
Does anyone have more current stats, which include ECC 6.0 numbers? We're on 4.6c preparing for an upgrade and our business sponsors would like to see any counts and types/sizes of customers on the various versions.