From a copyright-law perspective, it's not a slam-dunk that Curebit infringed any protectable aspect of the 37Signals design, as opposed to "stock" elements.[1] West Side Story has a lot in common with Romeo and Juliet, but the latter itself has a lot in common with a long line of still-earlier romantic tragedies.[2]

Some of the reactions here are fascinating. As a group, HNers prize the freedom to copy from others and the ethic of helping others --- but for some, it seems to be "I get to copy from others, but no one else is allowed to copy from me."

EDIT 1: That said, from a purely-practical perspective it's usually better and cheaper to steer clear of controversy (depending on how you think the PR scales will tip). Curebit might have been better off had it not copied what it did, not least because this controversy is sucking up management time that could probably be put to better use.

EDIT 2: Plus, stealing someone else's bandwidth by linking directly to their copies of images, etc., without permission does cross the line, if that's what Curebit did.

EDIT 3: Even copying HTML and CSS might not constitute copyright infringement if there are only a limited number of ways to express the same idea(s). This is even more the case if there's an optimal way to code a given set of ideas.

[1] http://en.wikipedia.org/wiki/Sc%C3%A8nes_%C3%A0_faire

[2] http://en.wikipedia.org/wiki/Romeo_and_Juliet