[R] convert princomp output to equation for plane?

[William Simpson]

I want to fit something like:
z = b0 + b1*x + b2*y

Since x, y,and z all have measurement errors attached, the proper way
to do the fit is with principal components analysis, and to use the
first component (called loadings in princomp output).

My dumb question is: how do I convert the princomp output to equation
coefficients in the format above?

I guess another dumb question would be: how about getting the standard
deviations of b0, b1, b2?

Thanks very much for any help.

Bill