What's the deal with "stealing" lessons?

Just a random thought I had the other day... what's the deal with one teacher "stealing" or more politely, "borrowing" another teachers lesson plan? I don't mean that they'd do so in a negative way, so I guess stealing is the wrong word. But basically, is it okay for teachers to use lessons written by and implemented by other teachers?

We have seen some really great lessons this semester, in our class but also (hopefully!) in our fieldwork. I know there are certain lessons (or unit ideas) that I could totally see myself using in my own classroom... is this a bad thing? Similarly, if I see a really great idea for a lesson on the internet, , is that too taboo to use in an actual classroom? Or what if it is something that a contemporary artist is currently using in his or her work? Of course in that last situation just teaching kids about that artist would kind of solve the problem, but still, where is the line drawn between borrowing someone else's good ideas and actually taking their lesson plan?

I'm sure this happens all the time in the world of art ed, and probably education in general, because of how easily word spreads about really great teaching methods. But is it necessary for teachers to appropriate or change the lesson they borrowed from someone else? Of course it will need to be altered minutely to make more sense for your students (always remembering-why these students, in this place, at this time?), but would it be wrong to take another teachers lesson if I find it useful and relevant to my class? In other words, would I have to (and how could I) change someone else's lesson and use it in my class without feeling like I stole it from them?