Is it unethical to look if someone tips their hole cards?

[flytrap]

We've all been in this position. Our opponents don't properly protect their cards when looking at them, and by glancing over we can catch a peek at what they are. If I know someone is tipping them, is it unethical to look when they are looking, or is it a welcome edge? It is especially tempting when its a live one that is steaming or otherwise out of line. It would be impossible to not make use of the information once it's aquired, but am I wrong to purposely glance over when my opponent is looking?