I have been arguing with clients for the last 2 years against purchasing expensive SAN's and RAID solutions to run their Exchange 2010 infrastructure. I explain to them a mailbox server or a disk is not important. Set your infrastructure up so you can remove any server from the equation and allow users to continue to receive email!
I get IT professionals and and Solution Architects look at me like I'm an idiot when I say things like "you don't even need RAID on your cheap mailbox servers in many cases".
Before purchasing expensive storage infrastructure to run Exchange 2010 please read the following article:
A big thank you to Robert Gillies from Microsoft for taking the time to document Exchange 2010 storage and giving me some backup in my arguments with customers.