Weighted Buffers?

[/S=E.ODOHERTY/OU=R02F06A/PRMD=USDA.FS.X400/ADMD=TELEMAIL/C=US/ at sprint.com]

I have a friend with a question who is not on the network yet, but 
here is the question:
I need to know if GRASS can do weighted buffers, and if so what 
command(s) you use to do it.  Example:  a buffer around streams 
where if slope is greater than 40% and the ground cover is trees, the
buffer should be 200', but if the slope is less than 40% slope and 
the ground cover is grass, the buffer should be 100'.  Clear as mud?
Thanks for any input!!