Interesting question! I was talking to a friend the other day who makes her own baby clothes (out of old sweaters felted and resewn, old linen tablecloths etc; all very organic and eco and MDCish). She said she goes to baby clothes shops sometimes to see what styles are 'in', then goes home and makes them herself. The stores aren't losing a sale because she wouldn't have bought the items in the first place; she just would have made less 'fashionable' clothes herself at home. One could even argue that her baby clothes were 'free advertising' for the pattern styles; if people admired her babies' tops, they might be more likely to look for that kind of top in the baby shops. It never occurred to me to think she was an evil pattern-stealer.
Example two... I bought some fabric to make DD some tops and trousers, which will in part be based on some baby clothes my sister sent her from England. They won't be entirely identical, because my sewing skills aren't great and I can't actually figure out how the originals were done; plus, I want them to be slightly longer/lined differently/not need buttonholes; plus I don't have a serger; and so on. In fact, I realised yesterday I could probably make the trousers by looking at the bought ones and then adapting a bought-and-paid-for bloomers pattern I already have. So as an amalgamation, I think it's perfectly legit; but even as a 'direct' ripoff-altered-only-by-circumstances-beyond-my-control, I don't know that I would have had a problem with it (which is not to say there isn't one, of course, just that I hadn't thought of it). No sale lost for them, no money gained by me.
What about trying on wedding dresses to get an idea of what looks good? I did that; Mum made the dress. I would have bought a dress if it had been the right price, and we didn't copy any of the dresses specifically (plus Mum changed the pattern a week before the wedding because the top wasn't working right, which is another story and a sad one!); plus of course we paid our $20 for the privilege of trying them on. So, any thoughts on that?
I'm always interested in intellectual property discussions, as a Linux-using somewhat-anti-copyright open-sourcey person. I think things can get particularly fuzzy on the Internet, which people tend to see as a source of free, good ideas and shared information. (And free step-by-step photographed craft tutorials are, IMO, more or less the best thing ever). I wonder if there's some kind of protocol on how disseminated an idea has to be before it's considered fair game...