Found this whitepaper from Microsoft about how they do security in the cloud with BPOS, http://www.slideshare.net/ukdpe/how-microsoft-secures-its-online-services-whitepaper.
It's an interesting read for those new to cloud services, however having worked in a company that has being doing this for 8+ years with Hosted Exchange I know this case well. If you fancy a chat about security and reliability in the cloud it why not visit http://www.cobweb.com or call us on 01329 242500.
Daniel