On the public library level, to make it understandable for everyone from kindergarten to grade 12, and that sort of thing, you can borrow a book from the public library by downloading it to your BlackBerry—and then it's gone; you sort of borrow it for a period of time.
Technically, there are people who know how to get around a lot of these things, and that's where the issue of digital locks comes into play. There's no problem at all with borrowing a book in digital format for personal reasons. On what happens to it after it leaves the library...I cannot predict how a librarian will try to imagine some sort of technological way that someone will actually do that. Why they would do that, quite honestly, is another question.
Yes, you can borrow a book right now and make 500 copies, set up a booth on the street, and hope to make a commercial killing. But it's not likely to happen. The same goes for digital books that come out of libraries in that way.