Earned value management (EVM) is a very usefull method to assess the state of your project.
By calculating some basic values like Earned Value, Planned Value and actual cost over a timeframe, you can easily check whether you're on schedule or behind schedule and on budget or over budget.
I always found it extremely difficult to figure out what my 'earned value' was in software, when using a traditional phased approach. However since Scrum it has never been easier to it.
Interested in more, read my post on pmScrum blog: http://www.pmscrum.com/blog/2010/10/evm-built-scrum