From Picasso to Bob Dylan art has always drawn heavily on precursors. One person’s appropriation is another person’s homage.
To answer the question of whether someone has infringed, a court of law would apply the substantial similarity test. This is exactly what it sounds like. The question is, are the two works substantially similar? Unfortunately, this is a very subjective test, and it’s often unclear what would be allowed without testing it in court. It’s always important to acknowledge influences, and to be prepared to defend your use of them. Context matters to everyone. Creators may not all agree, but you must be willing to engage in that conversation.
So what it comes down to is that if you feel someone else’s work is too similar to yours, you need to be prepared to make that case.
The ease with which we move in the digital world from one medium to another causes problems. Copying a painting into a stained glass work is still an infringement. Infringement is not about having the same idea as someone else - great ideas take flight in similar ways all the time. Identical photos can be taken by different people without it necessarily being infringement. If you are a creator whose work draws on someone else’s work, you need to be prepared to defend the similarities between your work and someone else's, and speak to the originality, and the context of your piece.
A recent Canadian example is the story of an Ottawa painter who submitted her work to a watercolor society and won an award. Her painting then ran in a newspaper. Someone recognized the image, did some research, and discovered that the artist drew from two photos.
The Watercolor Society rescinded the award after it became aware of the genesis of the piece. Is it an infringement? You decide ... For another example check out the Toronto Star December 24th 2010. For a US example: Shep Fairey, and more go to Artforum. For the Canadian approach to infringement, see Section 27 of the Copyright Act.