Can I use a 1" stem on a 1 & 1/8" steer tube? I have a Nitto Technomic stem that I'd like to use on a '92 trek 930 with the idea of converting it from a mountain bike to a poor man's expedition touring bike with drops.

I found this shim that would seem to do the job.

Has anyone tried this? Is it a bad idea? I can't think of any reason why, but that's why I'm throwing it out here.
I've done the opposite (but drop bars before and after), and have a quill stem adaptor to use a non-quill stem with a threaded headset. My fork is 1", but the adaptor came with a similar shim for 1 1/8" steerers. I've also used quill stems with threadless headsets, but only with matching sizes.

I don't see any reason the product you link to won't do the job it's designed for though.

To replace a non-quill stem with a quill stem you'll need something like 
http://problemsolversbike.com/products/locking_headset_spacers, cf. "Threadless Without Spacers" at http://sheldonbrown.com/handsup.html#threadless

I assume you are replacing a 1 1/8" quill stem on your trek though?