In searching for the ultimate, safe cat tree, I found a very interesting company called Feline Furniture that sells modular cat furniture called Catnex. The furniture is made of snap-together plastic building blocks covered with very low-pile carpet and with hammocks or other pieces of cloth attached to the plastic frame.
I like the fact that this is all so modular and that it looks like the carpet won't easily be chewed off. That said, it doesn't look as solid and sturdy as I would want. The site claims that it's sturdy, but I'd be curious to hear from people who have bought it.