One gap-stop backup in case of GPS failure is the LORAN system. Popular Mechanics has a short introduction to the LORAN system which uses ground stations to geolocate targets. However, the LORAN system is being cut out of the new Obama budget.
While the threat of losing GPS in 2010 is receiving more attention than the actual threat there are plenty of things which could cause GPS failure. One would hope something so critical to keeping the geoeconomy running would be in a relatively stable situation with backups.