So what has this to do with Scrum? Well firstly Salesforce.com itself uses Scrum to manage all its own product development and they have seen a 60% productivity improvement as a result.
The difference between Scrum and other project management methodologies is that it emphasises managing implemented system functionality against a fixed time period whereas an approach such as Prince 2 focusses on managing time against a fixed functionality scope.
The challenge that both approaches face is that project estimation is uncertain but in the first case you get a fixed price but a risk that not all the functionality will be implemented. In the second case you might get all the functionality but at the risk of cost overrun.
The benefit of using Scrum is that you can decide at the end of the fixed period whether the return that you would get from the missing functionality is worth the extra cost. The reason that Salesforce.com makes this possible is that unlike other types of software infrastructure (particularly on premise databases) you will have a working system. It may not do everything but it will function and you can then evaluate it against the real world requirements.