osumc2014, just put it on your original AMCAS.
I had a very similar situation this past summer. I submitted my AMCAS application 1 week after starting a summer research position. On the AMCAS you get 250 words per activity, so I explicitly said that I was just starting a research project, with such and such program studying X, Y, and Z and explained why I was interested in that subject matter. Again, I made it clear that I was just starting the research. It actually made a fantastic talking point during interviews because everyone was like, "I see you were just starting research in bioinformatics when you submitted your application, can you tell me what you ended up finding?" Of course, be careful, because if it turns out that you have an awful summer experience they are going to ask you and you are going to have to say you left your internship half-way through or whatnot, but if you are confident you will stick it out I would say write it on the primary.