this happens to me, too. i take an attitude of leniency, because i get inspiration from others as well. however, if it's an exact rip-off and it's threatening to your profits, you can take the legal route:
call the copyright hotline: 202-707-9100.
request the form "va" for visual arts. there's a small fee and an application to register; you also need to send a couple photos of your work to the library of congress, but this'll protect your creations for your life and give you grounds to get an injunction against the infringer.
(this info. is from wendy rosen's book, crafting as a business)
Coppyright is funny. We were just learning about it in my legal aspects class, and it made me think of stuff like crafts and what not, because you can't coppyright a method. There fore, if you come up with some new way to do something (make a shirt) and some one coppies it, you cant sue them for any kind of coppyright thing.
But dose design fall under that. My simplicity pattern sitting next to me has that symbol coppyright on it...but how can that be.
My point being, I think it would be hard to prove, and hard to take legal action.
Also, you CANT take legal action unless you register whatever you are trying to sue for at the US coppyright office. I know that.