Disclaimer, IANAL and all that BS

I think that creative types severely underestimate the difficulty in enforcing any sort of infringement, whether direct or inspired. How the clones become clones is a very genuine question. The same issue exists in the art world with regards to copies of famous paintings (often where they look indistinguishable from the original), there's even an entire set of case law dedicated that issue, and frankly, it can go either way.

If it's that difficult to decipher (because as Otto said, this is not a black and white issue). The biggest red flags in almost all cases are any of the branding distinctions. The Twitter logo or font (if it's a special font and you are using it without a license), the Facebook logo, sometimes even certain colors can all be way too close for comfort. When it comes to actual layouts, however, assuming the CSS isn't just a straight copy, I think that trying to get someone to take down a website because it shares the same layout pattern would be pretty difficult. Especially since most sites do employ many of the same characteristics thanks to stuff like 960.gs and other CSS frameworks.

And of course, you can't talk about one side of the issue without talking about the other. While I think we all agree that people that create and sell rip-off designs (and I'm talking verbatim copies, not inspired by stuff) totally suck and should be shamed, if you take protection too far, it can get flat out stupid. True story as a counter-example: T-Mobile (division of Deutsche Telekom) sent Engadget Mobile a cease and desist notice in 2008 regarding the use of the color magenta in the Engadget Mobile logo. Nevermind that this logo had been around for a long time or that it was s different hex count than T-Mo's magenta, they said, "we are the only telecom related website that can use magenta, period." Weblogs Inc. went Magenta network-wide the next day to show support. Nothing ever came of the complaint and then T-Mo jumped on Samsung or Motorola or someone else who dared use magenta. Again, with typical unsuccessful results.

So on the one hand, totally derivative designs are problematic to do and have lots of potential (though realistically unenforceable unless they are taking your creative assets like images or trademarks), on the other, if you go too far down the rabbit hole stuff can get just stupid.