The short answer: It depends, but typically user interfaces (UI's) are not legally protected.
You should not be able to patent obvious, non-novel ideas (but sometimes the patent office fails to verify the originality of patent applications, sadly). You also cannot patent ideas that are already in general use in the industry, or a visual appearance (a look'n'feel). All common controls (menus, scrollbars etc) are already in use in the industry. Thus you can't patent a whole user interface (which is a collection of controls, with a certain look'n'feel applied).
If a UI is a 100% ripoff of someone else's, with the exact same colors etc but just a different company name, then I'm pretty sure the courts would rule against the infringer. But it would probably not be based on copyright or patent law, it would more likely be based on rules against impersonation, phishing or brand infringement.
One thing to note: While you typically can't protect a full user interface, you can protect single elements of a user-machine interaction it if they are deemed novel. For example, there are patents on ActiveX controls loading in browsers, and the shopping cart metaphor when used on a website.
Where is the line drawn with regards to UI ripoffs? There is no way to answer that really, it depends on the jurisdiction, the protection in place (registered trademarks), the intent of the infringer, and the legal posturing of the infringed. The latter is especially important, trademarks etc are not automatically upheld by the police or courts, the company that is infringed upon must itself act to protect its interests.
If you're in doubt in any way, or if you're doing something outside the commonly accepted, then go see a lawyer who is specialized in intellectual property protection.
Steve Blank recently wrote a good overview of intellectual property protection, with links to some good papers and articles (US centric). It's well worth a read.
Disclaimer: I'm not a lawyer, and the above should not be construed to be legal advice.