When I interviewed for my current job (research lab) last June, I did mention I was hoping to start med school in the summer/fall of 2007. That said, I was able to give at least a year commitment and the particular lab I was applying to was used to having someone hold the position for a year or two before moving on. I was above and beyond glad I had been upfront about applying because they were incredibly nice about letting me arrange my weekly schedule to accommodate interviews....I know people who tried to not tell their job they were applying, and figuring out how to get time off for interviews was a nightmare for them.
So for anyone reading this thread, if you're just about to *start* your application cycle, I would definitely be up-front about applying to medical school.
For the OP, personally I would probably dodge the research job altogether and go for a laid back job that pays enough to get you through to the start of school and just have fun. If you're all about money and near a city, there are often high-paid temp positions available at banks and other big companies....a short-term gig doesn't have to mean you don't get paid well. The problem with research is that often it takes a few months to train the person and have them settle in....and it's not really fair on the lab if you're just going to take off in a few months. Also, what benefit are *you* going to get? It's not enough time do really do anything of significance, and after you bail on them they're probably not going to be a great reference/LOR.