Sorry, I am getting to this thread late. It is getting long so I thought it might be interesting.
The problem, as I see it, is that it really isn't a cross. It's just two beams from a building that got welded together. Some Christians are saying that it is a miracle from God, proof that the God of Christianity is watching over everyone because the Christian God caused the cross to form.
It's like a piece of toast with a burnt bit in the middle that people say is the face of Jesus.
So it really depends on the wording on the museum display - if it implies that it was created by a miracle or if it points out that they are just two crossed beams from a very big building that was full of crossed beams, that some people think is a cross.