For those advocating TCAS: Bear in mind it is designed for transport category controlled airspace separations, traffic density and performance. I suspect (but don't know never having looked at it for the GA role) that it would be a lot less use than many might think. If nothing else I can imagine the datalink freqs getting utterly saturated.
Everyone has their own opinion of how useful it
might not be in theory, but those of us who
do actually use it in Class G don't ever want to be without it.
As I said before, I've been flying with it for VFR / IFR in Class G for almost 20 years now (and the previous 25 flying without it) and I do know how good it is. It's not the perfect answer, and no system will ever be, but it is far better than flying without.