I've seen several sites, other people copied from the sites I made and I have some thoughts about it. Sorry, it can be offtopic here, cos I'm not offering a way, how to determine possible design theft.
So, my opinion is the following. There are design trends and styles. If site was made in the same style, like the other one - is it stealing or just keeping the trend?
As webdesigner I was mad, when I saw someone stealing my design, however, when I've seen, like someone just developed my ideas and created something own, based on them - I was happy.
Anyway, be aware that design wasn't stolen, otherwise you may spend unaffordable (despite of the result) amount of time in court room later.
p.s. and, guys, you have no idea, how many web development contracts I declined, after clients tried to force me use someone else's design, saying F%$&CK THEM, after my comments about copyrights. But there were ALWAYS some developers, who did that for those former clients of mine. Mainly - russian and eastern european outsourcing. Anyway, I respect work of my colleagues in webdesign (you know, webdesigns is more, than knowledges of MS FrontPage and Photoshop
) and I don't violate their copyrights. So I usually expect something similar in return. Damn, it will be good, if everyone will follow this simple and fair rule. Amen :)