Here is a link to Using the Waterfall from Josh Kopelman's blog, "Redeye." This is really worth a look. (You'll find an actual example of the spreadsheet there as well.)
One of the interesting questions to ask in a funding discussion is "how have you done versus your forecasts at various stages over the last X years?" It gives the investor a view of the dynamics of the market and the company's reaction to them.
This spreadsheet encompasses that process in an easy to read, straightforward way. I think I'm going to ask our portfolio companies to start using it. It incorporates the quarterly re-forecasting paradigm as an expectation rather than an idea, and prompts a monthly - or more often - look.