Why the subdomain? Can someone explain why a lot of app are doing it? Personally, I avoid using an app that uses a subdomain.
See also http://stackoverflow.com/questions/918971/when-should-i-use-... and http://www.mattcutts.com/blog/subdomains-and-subdirectories/
It also seems to work pretty well for all the 37signals apps and and few others that segment this way.
But, talking about them, why 37signals are doing it? They have "normal" web applications that don't require that kind of separation to be efficient.
They probably do it for the same reason we do: easy to direct traffic to different DBs and VMs this way, instead of waiting for the request to come in and direct it some other way.
There might also be an added value to the business/account seeing their name in the url... Not sure about that one though.