Are any A Class cats manufactured in the US?

Yes,
http://groups.yahoo.com/group/LR2ACATS/
http://www.bluestreakboatwerks.com/
and http://www.guckinc.com/ is making the A3 under licenses from www.morrellimelvin.com

I noticed from the Class web site that there are nearly thirty designer/constructors around the world. Which boats are most popular here in the US? In other words, what builder would most likely be around in ten years to supply spare parts and other support?

There are quite a few, but the top boats seem to be the A3, Flyer MKII, Bimare, Marstom with some newer platforms like the Evo becoming popular. It really depends upon the sailor as to what he likes. Lars Guck who now builds the A3 has been a supporter of that platform from early on. Glenn Ashby AUS is on the Ashby Geltech Flyer. Both are at the top of their game. Hope that this helps a little.