"I recently asked one of our developers to draw up a design for a specific component. After a few hours he returns telling me that he'd solved a very similar problem a previous place of employment and that they had developed a "neat" solution. The developer then became concerned that a ground-up re-implementation of these design patterns and principals may infringe on the other companies intellectual property or breach some copyright laws. This developer is talented and experienced that's why we hired him. The question is, at what point does 'drawing on experience' cross the line and invade others IP?"
The sad part is not the answer, but that the question needs to be asked. As one of the posts on Slashdot says, the answer depends on too many other variables to give a reasonable answer.
But before you get too upset, in general the answer is the engineer is mostly free to do as he wants for his new employer, with the exception of anything that may have been patented in his old solution, which happens to apply no matter what. Copyright is a null issue here as long as he doesn't pull old source code owned by the previous employer.
On that topic, I'm trying to build myself up a library of Python code that will be useful to me in the future, which I'll happily license to an employer in an unrestricted, but non-exclusive manner. The downside is I have to watch what I sign now; I can't afford a contract that tries to take ownership of everything I do, or worse, that I've ever done. You'd think the benefit of hours of programming not on their clock would be enough, but somewhere in their education, lawyers are apparently trained to ask for the moon, and not take anything less. (Been there. Didn't sign. Their loss.)