One month really isn't that long in the grand scheme of thing. If it were me, I'd give it a full three months before making a decision. However, you should listen to your instincts. If you feel the job probably won't be fulfilling, move on. Keep in mind, as a junior developer, you have to "pay your dues", which often means doing grunt work, stuff more senior developers don't want to do and will hand off to junior developers.
Having IBM on your CV is a good thing. I imagine they treat junior developers as truly junior, though. A smaller company may be more personally rewarding in a faster time frame.