The first solution was submitted by Benjamin Hall.
It passed all of my tests, and be downloaded here (you will need to choose File>Download).
The second excel formula was submitted by Patton Richard.
It passed my tests as well. You can download that spreadsheet here.
UPDATE: A third solution was passed along that allows you to calculate the rating in a single cell, without the need to have a bunch of hidden cells in the background. This is very useful if you want to drag it across a series of rows where you have attempts, completions, yards, touchdowns and interceptions already filled in.
This solution comes from Jason Reilly
So there you have it. Link to this page from your sites, and share the link with others so that future football geeks may have an easier time finding this, and less time using annoying online calculators. Geeks unite!