A friend of mine has an interest in GPS in mobile phones - most new mobiles in the US now have a GPS chipset.
He sent me this:
"GPS is increasingly at risk from a rising noise floor caused by anything from electrical pylons, to WLAN, to faster chips in computers, and many more sources. It is a very weak signal - some 10,000,000 times weaker than GSM for example. It will mean GPS performance gets worse over time, as opposed to the normal trend of improvement."
Clearly an aircraft, when airborne, is some distance away from most of the low power sources, or passing through their influence fast enough not to be affected, but it's worth bearing in mind.