I just came across this BBC article on the use of email as a database. The basic premise of the article is that, as users store more and more information in webmail clients for later reference the market share of the various providers is shifting to those that provide more storage space and faster searching. I like this quote from the Hotmail guys:
"If we find people are using Hotmail as a dumping ground [for information] and not being remotely organised, we will develop the product."