I was lucky enough to take a class in college where the prof understood and embraced this idea. After all, he was teaching us to be engineers rather than coders. On the first day of class he told us in no uncertain terms that, if we found some code that would be useful in our projects, we were free to use it as-is provided we clearly noted where it came from.

He even told us a story where, in a previous year, he (inadvertantly) gave out an assignment that was almost identical to an assignment given at another university. Many students took posted solutions from that other class, cited the source, and turned it in. Given the prof's policy, he had no choice but to accept the programs as valid and correct. As he was quick to point out, it puts a much larger burden on him to come up with more creative assignments -- which isn't a bad thing.