ADS-B/multilateration is working well for the approach at Innsbruck, is it not? I think it's a technology that holds great promise. However, at least on this side of the pond, I don't think there's much threat of it replacing primary radar. Our ARSR-4s are part of our Joint Surveillance System and, as such, are air defense assets. Sept 11 reminded everyone that we still need to be able to track primary targets (something ATC knew all along) and the plan to scrap the old ARSRs in lieu of SSR-only sites was abandoned. Depending upon the ADS-B model without primary radar just takes us farther down that road. So, I see a great deal of utility in the use of ADS-B in a radar-like setting as an additional tool, but to put all of our eggs in that basket is just foolish.
RE: GPS jamming... ALL ATC technologies are susceptible to interference and the trick is to recognize when somebody has mucked with the hardware. So we augment (for instance, through things like a Ground Based Augmentation System) and watch closely for signs that something has gone wrong. When it doesn't look right we stop using it and move to another system (you do it in the airframe and we do it on the ground all the time). So, yes, if somebody DID jam the GPS signal things would go "sour," but if you mean that aircrews would continue to fly a Cat III GPS approach (sounds fun!) after the jamming began, I disagree.
Dave