At some point everybody in business (and I mean it should be everybody) should be wondering if they could gain some leverage from "the Cloud". What you are really asking yourself then is "what can/should I outsource?" You could also think of the answer to the above question as justification for outsourcing something rather than servicing it in-house. At least this is how I arrived at this question.
What I've concluded is that anything and everything that is not your core business can and should be outsourced hence shifted into a Cloud service if one exists for the function/skill/technology in question. I've heard that some VCs insist on the use of cloud services.
you core business = your secret weapon/differentiators/intellectual property => you should be in full control
For example for most companies running an email system such as Exchange or Lotus notes in-house is not a business differentiators. Rather a back office system or a tool that is required to conduct business like a pen or electricity, a means to an end. The same applies to phones, PC O/S, server hosting, CRM software, and ERP software.
There are many more questions to be asked but most of these are to answer the question "which cloud service?", that's for another post.
This is obviously a simplified view on the topic also directly or indirectly the sales pitch for many cloud service providers. Here are a few links with further information.
Just seen a really good post The end of bespoke by Matt Ballantine where he goes into a lot more detail although coming at it from a different direction.