[R] Random Forests: Question about R^2

[Dimitri Liakhovitski]

Dear Random Forests gurus,

I have a question about R^2 provided by randomForest (for regression).
I don't succeed in finding this information.

In the help file for randomForest under "Value" it says:

rsq: (regression only) - "pseudo R-squared'': 1 - mse / Var(y).

Could someone please explain in somewhat more detail how exactly R^2
is calculated?
Is "mse" mean squared error for prediction?
Is "mse" an average of mse's for all trees run on out-of-bag holdout samples?
In other words - is this R^2 based on out-of-bag samples?

Thank you very much for clarification!

-- 
Dimitri Liakhovitski
MarketTools, Inc.
Dimitri.Liakhovitski at markettools.com