No, the GPS is not a reliable backup device. The issue is not latency. Aircraft GPS systems generally update faster than 1 Hz, which is plenty fast for most things. The real issue is that GPS gives you the ground speed, and not the airspeed. The wind speeds at 30,000 ft can exceed 200 knots [1]. Therefore, depending on the wind speeds and direction of travel, your airspeed and ground speed can be vastly different. Of course, it is theoretically possible to correct the GPS groundspeed using weather data (would have to be downloaded from external sources) to correct for upper winds, but this is not reliable over much of the surface of the world.

[1] http://en.wikipedia.org/wiki/Jet_stream

This is all true, but except for stall avoidance, when navigation or landing you care about ground speed. The old ILS doesn't know airspeed or ground speed.

GPS coupled with a laser gyroscope can provide data at KHz frequencies.

It is better than nothing, but the speed through the air is the most critical parameter for an airplane, and GPS measures speed over the ground. If a sensor goes out, the computer could assume that the wind is constant, and display an estimated airspeed, but the probe in this case iced over, in this case because of a thunderstorm. The wind is all over the place in a thunderstorm, and it wouldn't be very useful.

GPS is used for all these things. It's even used for automated landing systems: http://www.freeflightsystems.com/gps_landing.htm

I think the intentional lack of resolution in civilian GPS is in part because it is not intended to be an air navigation back-up system.

I was under the impression that the "degraded civilian" properties of GPS have been disabled for some years now.

I believe that IFR-certified GPS systems have the added capability of factoring Differential GPS. This is an additional GPS beacon (aside from the satellites) originating from the airport you're landing at, which have a location that's very accurately known.

High-price civilian GPS receivers are capable of this -- except you rarely have access to such a beacon. I mean, the restaurant I'm going to eat at isn't going to provide my little TomTom with a differential signal. With aviation, it's important enough that high-traffic airports provide the DGPS beacons.

This isn't true. IFR certified GPS units generally don't have Differential receivers. The main reason for this as far as I can tell is integrity. To be IFR certified, the receiver has to be able to tell when a satellite is giving bad data. In the older units, this was done with RAIM, which basically compares what is coming in from all of the satellites, and determines if all of the satellites are providing positioning that is relatively consistent. DGPS is a relatively simple system, and there's no way to check the DGPS signal for integrity. Without the ability to check for integrity, the additional accuracy is useless, as the design of instrument approaches is based on worst-case scenarios.

Now newer IFR certified units do have WAAS, which is similar to DGPS, except the signal is transmitted from a satellite, and is based on measurements from a couple dozen ground stations, the correction interpolated in the unit.

There has been a proposal to implement a system called LAAS for a number of years, which would be like DGPS, but would have the integrity checks that are required for aviation. However, it never really got off the ground.

As it stands, with WAAS, you can be guided to 200 feet above the runway, but this is not as good as the old analog radio ILS, which, if setup in a sophisticated way, can take a plane all the way to the surface.

Yes, but I believe they still reserve the right to restore them whenever they choose.

From Wikipedia (http://en.wikipedia.org/wiki/GPS#Selective_availability):

On 19 September 2007, the United States Department of Defense announced that future GPS III satellites will not be capable of implementing SA,[59] eventually making the policy permanent.[60]

Thanks, I did not know that.

Aircraft with Wide Area Augmentation System (WAAS) are certified to use GPS as a primary navigation source. [1] [2]

[1] http://www8.garmin.com/support/faqs/faq.jsp?faq=221

[2] http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System