Jack Stein,

Those are almost exactly what I've been prototyping. I built this
cart:
http://www.scrgeek.com/woodwork/storageSheets.html

I beefed it up some -- used EMT instead of PVC, added some lateral
support, etc. It holds a ton of material, but at that capacity it's
not "mobile". The multipurpose cart you made is probably more
practical. I have 10 sheets of birch/maple ply that I need to
store. I just need to realize that it has to lean up against a wall.

On your pipe rack -- did you face-glue two 2x4s? How did you drill
the proper size hole? Did you secure the pipe into the hole?