I hate to be the bearer of bad new but...maybe. A reading of the Computer Fraud and Abuse Act could make robots.txt legally enforceable. And given the government's approach to CFAA cases a very aggressive interpretation, under the right circumstances (for example, when it provides evidence that the scraper knew that scrapint was not authorized), seems like a real possibility.
Among the many other things CFAA criminalizes, it makes it a crime to "intentionally access[] a computer without authorization or exceed[] authorized access . . and thereby obtain[] information from any protected computer;"
A "protected computer" is, among other things, a computer "which is used in or affecting interstate or foreign commerce or communication." That would probably cover just about any server on the Internet.
The interesting question is are you violating the CFAA in a way that will cause the executive branch to exercise their discretion to prosecute, and moreso, is the CFAA even constitutional.
From a moral point of view: robots.txt states an intent, and intentionally ignoring that is not a nice thing.
And often, "not a nice thing" translates into legal action. So, if this matters to you, you should find an expert :)
As a lawyer asked inane questions by friends, a common answer is: "I am no sure if its legal to do, but I'm absolutely sure you're a asshole if you do it."