Last Friday, the Exchange team published the initial version of an Excel worksheet describing the scalability limits and recommendations of Exchange 2007 SP2 versus Exchange 2010. It shows to which Exchange version the limit applies, the area, the limitation itself, a description of the underlying issue and where possible mitigations to increase (or lower if you want) the (default) limit. Note that it not only describes the Exchange Server software, but also limitations caused by the underlying Operating System, off- and on-premise usage and running in large organizations. Great info for sizing large scale implementations and deployments!
You can download the Exchange Scalability Limits Worksheet here. The Exchange team welcomes comments and feedback.