Just a note on my recent update to the r.plane docs...
I noticed that r.plane dips the northern half of the plane in response to increasing dip values; this was unmentioned in the docs, so I added it.
The manpage mentions that increasing azimuth values rotate the plane to the east (i.e., azimuth=90 means East), but I couldn't confirm this even after much experimentation.
Instead, the script rotates the north portion of the plane counter-clockwise through increasing azimuth values (azimuth=90 means West); the script message output confirms
this as well by printing:
"Raster map <test_65@PERMANENT> generated by r.plane at point 349345 E,
5035380 N, elevation 0 with dip=-10 degrees and aspect=65 degrees ccw from
north.
So it looks like only the doc was in error, unless I've somehow got my directions flipped around 
~ Eric.