Have a look in src.contrib/OTHER/s.to.rast in Grass4.1.
There is the program you are looking for. Katarina ( katarina@fmi.kth.se )
wrote this module and you can a copy from her as well if you don't have
4.1 yet.
Lars
Lars Schylberg Email: larss@fmi.kth.se
Dept. of Geodesy and Photogrammetry
Royal Institute of Technology (KTH) Tel. +46 8 790 86 33
S-100 44 STOCKHOLM, SWEDEN Fax. +46 8 790 66 10