Anyone know if all 737-800 use the same alimiters or has AA spec'd
a different model/version.

http://www.atwonline.com/news/story.html?storyID=16245

Regardless, there is more than one altimeter in each aircraft, and there are other systems pilots use as well.

(N.B. Though the OP is concerned with the altimeters used specifically in AA's 737-823s, the altimeter faults seem to have occurred with Turkish and Qantas and are best discussed as an aviation / travel safety topic - the thread will relocate to the Travel Safety/Security Forum. Thanks! /Moderator)

OT, but how useful is GPS as a backup to altimeters? is it too slow?

I'm not sure.

GPS systems (all mapping systems) make assumptions about the the shape of the earth's surface. They generally assume that the surface is "smooth". Each system uses a different "geoid". And the differences while small can have an impact.

I've done a lot of blue water sailing -- there have been may times when a GPS unit has shown me at 150 feet above or even below mean sea level. (An aside, GPS is a very bad thing for coastal navigation -- one's eyes and a knowledge of how to use a chart are far better...)

But, +/- a few hundred feet should not be a problem at 29,000 feet. At the height that airliners are spending most of their time they should be OK.

But if east bound and west bound flights are supposed to keep 1,000 feet of vertical separation this could easily eat into that.

The fault is in the radio altimeter which is only used below 2,500 ft. It is used by the autopilot for approach and landing and is used for the ground proximity warning system. Had the pilot not disengaged the autopilot, it would have landed the plane 1,950 ft. above the ground based on the erroneous reading. As far as inflight navigation is concerned, that is based on the normal pressure altimeter so the malfunction would have no effect.

Above 18,000 feet, altimeters reset to standard 29.96 in hg. Below 18,000 feet, the altimeters are set to the barimetric pressure at their destination airports barometric pressure as provided by ATIS recording (Airport Terminal Information System).