The sky has no limit... Or, does it ?

I've just heard an interesting story from my biochemistry class. Once upon a time, there was a scientist who was really interested in a flourescent capability of jellyfish. Thus, he investigated about the gene encoding the protein that caused the jellyfish glows in the dark. What so special about this protein is that you can use this protein to trace other unrelated proteins. This can be done by putting this protein under the same promoter as other proteins of interest. To make a long story short, this protein is very popular so that it has been widely used in science to localize the expression of other protein in living organisms. In addition to that, in Japan some scientists have already used this protein to make a rabbit glowing in the dark. According to my teacher, it really happened,... So, where is a limit for a scientist to manipulate another living system ? Or, shall I say: transgenic organisms ? And, who's going to define that limit anyway ? Another scientist ?