<p>Is it legal to more or less "copy" (or euphemistically, be very "inspired by") the idea and UI of a copyrighted program? This question popped in my mind when looking at "<a href="http://culturedcode.com/things/" rel="noreferrer">Things</a>" which is a Mac-only program.</p>

<p>How legal would it be for someone to make a Windows version of this program with a basically identical UI design and feature set? Where are the legal boundaries of what counts as copyright infringement?</p>

<p>Unless you specify otherwise, I am assuming an "IANAL" for any answer, so you don't have to write that. ;)</p>

Specific elements of a design can be copyrighted. Apple won the "trashcan" icon, but effectively nothing else during the Microsoft/Apple lawsuit over a look and feel. As long as the code is entirely your own, the graphics are your own (and yes, copying too closely a
specific
graphic element like the trashcan can be a problem) and you aren't doing cloning elements that are trademark/tradedress related, you should be within the law (in the US).
Of course, that doesn't mean that someone can't
sue
you. Meritless lawsuits are started all the time for intimidation. Also note that patents can cover the interaction of the user with a UI, so copyright might not be your primary problem in some cases.