IO540 - sadly, there is a proposal to phase out VORs - by something like 2014. The plan is to go to DME/DME and GPS, presumably linked with the ADS-B equipment that we will all have to carry.
Note ADS-B, not Mode S. The inference (not stated clearly anywhere) is that we'll all have to buy Mode S kit by 2008, then throw it away and go to ADS-B a few years later.