Viaweb got copied a lot, and now YC does. I've learned not to worry about it. Clones rarely amount to anything. First of all, anyone who has to copy you is probably not that good. Someone really good wouldn't be able to copy someone else, even if it was to their advantage; they'd be too proud to. Second, people can only copy what's visible. They can't copy the half of the project that's in your head, and that is probably the more valuable half.

Be flattered!

If your site can be cloned, launched AND cause a serious dent in your business, you need to think of better ideas.

I wouldn't be worried. Cloning websites never goes as well as the poster of the RFP. The poster will be playing catchup without differentiation. You have an advantage, just keep innovating and developing it. Even if there is a really good developer that can clone it, it will be hard for them to clone, catch up and surpass.

Almost inevitably, shortly after I launch something that proves reasonably popular, I see an ad for someone wanting to clone it on Scriptlance for $200.

You bid on the job. Eh? ;)