Rather than regurgitating the comments on this blog, I figured I would simply share it: https://ethereum.stackexchange.com/questions/379/what-is-a-sidechain
I don't honestly know enough about SMTs, but it's possible that they could position themselves into the "side chain" slots I mentioned. It likely depends on how they're built. It seems like you could use side-chain and SMT interchangeably, but at the same time it could be designed in a completely different manner.