ADS-B is very nice. It has been in use in one area of Alaska since January for ATC purposes. Unfortunately, it is not as straightforward as as you say. There are no internationally agreed protocols or operational specifications yet. VDL (VHF Data Link) Mode 4 is one data link proposed to do it, there are others. The data link in the Mode S specification (1090mhz) can do it and the link they use in Alaska can too...problem is it is also a piece of the radio frequencies used by military JTIDS data links.
It is also not a replacement for TCAS.
As far as cost goes the Alaska Capstone project bought 200 ship sets of GA avionics for I forget how many million dollars but I think it came out to $15 or $20K per airplane plus installation. This avionics fit isn't enough to do approaches and you need GPS local and wide area augmentation systems to get the navigation data to the point that you can do precision approaches.
It is a good thought though.
[ 01 November 2001: Message edited by: RATBOY ]