How mobile is your shoulder? Can you endure miles of relaxed, effortless freestyle? If not, scratch the race. If yes, do as Terry did after his shoulder surgery. See blog post: 'How a Shoulder Injury helped me swim better than ever,' by Terry Laughlin, Posted on Apr 09, 2009 at http://www.totalimmersion.net/blog/H...than-ever.html
I deal with a sore shoulder, an old injury that pre-dates my swimming efforts by several years. It forces me to find my most efficient place in the water.
Careful progress is important. Work it too much sore and you will end up with a case of tendonitis that could require years of rest to heal. Ice after working it and deep tissue massage. Don't just swim--balance strength in your shoulder through physical therapy. The Total Gym or elastic tubing are good for covering the full range of motion.