TidBits published published an interesting article about the GPS system, it argues that the accuracy may start dropping next year as the chance is rising that the satellite constellation drops below the required 24 satellites. Some of the GPS satellites are almost two decades old and are in need of replacement, action is needed to ensure the accuracy of the GPS system remains high.
Delays in launching new satellites - the next one is scheduled for a November 2009 launch, almost 3 years late - could be problematic if the older hardware starts failing. The GAO has calculated - using reliability curves for each operational satellite - that the probability of keeping a 24-satellite constellation in orbit drops below 95 percent in 2010, and could drop as low as 80 percent in 2011 and 2012. And if the Air Force doesn't meet its goals for the next-generation GPS III satellites, the probability drops to around 10 percent in 2017. (The GPS III satellites bring with them new features, including new military and civilian signals for greater accuracy, particularly in urban environments, plus higher power for current civilian signals, which will help existing GPS receivers.)
Even if the satellite constellation drops below 24 satellites, that doesn't mean that GPS service will fail altogether. It does mean that the level of accuracy that both military and civilian users have become accustomed to - which is actually higher than promised - may degrade significantly.