This has been covered by other forums pretty thoroughly but I'll add my 2 cents. I had my 98 Tacoma bought back by Toyota. The passenger side frame rail was crumbling. I had no rust on the truck anywhere but the rear bumper. I had no idea the frame was gone.
Fast forward, I had a 2001 that had the frame replaced in 2012 by Toyota and I just traded it in on a 2006 that already has a new frame under it. If you live in the "rust belt", do not buy a Tacoma unless it either has a new frame already, has a very clean rust free frame or you buy one with an existing undercoating done by Toyota, which means that truck will get a new frame in the future if it fails. If I lived in Florida, I would not be worried unless the truck came from Vermont originally. If it lived its whole life in the south, I would certainly look the frame over carefully, hit it with a hammer a few times in a few places but would worry much less.
Anything on the used market here in NJ is a rust bucket. Just depends how rusty. I jumped on the 2006 truck because it had a new frame and only 73k on it. Brand new truck basically.
-jorge