Did you decide on sonars or laser, I have worked with a smaller laser range finder that only works Indoors,
http://www.acroname.com/robotics/par...YO-LASER1.html
the price tag is still quite high but much more reasonable than SICK for the amount of range detection.
I am interested in how to turn this sensor into an outdoor sensor. I have read papers from the IGVC & Darpa competitions about teams using Indoor SICK lasers in an outdoor environment, therefore I know indoor lasers outdoors is possible. I am also interested in learning about more lasers available on the commercial market. Lecia and other geo-survillence companies usually have this type of equipment.
I've put some of my thoughts on this subject into a more streamline and concise (unlike this forum post) webpage on Wiki Trac (great software ! )
http://ras.ece.utexas.edu/trac/wiki/LaserNotes