I think it's a great thing for start-ups or smaller companies with limited IT resources. Using EC2, Azure, and Google require very little upfront capitol investment and would allow them to focus on their business vs their infrastructure. For the smaller folks or new companies which don't have an established infrastructure I think Google can be a great way to go. One caveat to that is financial or billing groups - the complex formulas and spreadsheet capabilities are not mature enough in Google Docs (from what I hear). I know MS Office Web Apps are out there though and may be more fully featured...Not sure on how they license it.
On the flip side once companies become large enough offloading some of these functions to a cloud provider makes sense. I know in my case I've been running the numbers. Exchange, with its HubTransport/CAS/Mailbox server breakdown can end up consuming a crapload of system resources (not to mention physical servers) for a relatively small number of users. Tack onto that the storage needs, backup software and resources to manage those components and the argument can be pretty compelling.