funny you should mention...Just did a bit of foaming on my S13 last night. If done properly should yeild awesome results as far as chassis rigidity is concerned.
I too was curious so i did a lot of research into chassis foaming. Found that some spec miata guys do it. And a few DelSol/ EG chassis honda owners do it with what they described as awesome results. Then there was the SCC article on it a while back, great idea...poorly implemented.
Its all about the density of the foam you use. The SCC article used a gas charged delivery system using 2lb/ft^3 density foam. The stuff that everyone else I've read about doing it used at least a 6lb/ft^3 foam. Up to 15lb/ft^3 is available though.
Foaming probably isnt as good as a cage for safety in a race sort of situation. Its in my opinion a great alternative to caging for a street car. But probably best implemented with a cage as well. Foam will allow you to brace area's that a cage cant get to.
Links:
http://www.sportcompactcarweb.com/pr..._300zx_part_5/
http://www.smooth-on.com/foams.asp
http://www.uscomposites.com/foam.html