I always felt myself if I had a cold or was stuffed up it was a good thing to workout or go for a running to break it up, and it looks like the research supports it.
I found a article in Runners World where David Nieman, Ph.D., who heads the Human Performance Laboratory at Appalachian State University, and has run 58 marathons and ultras, uses the "neck rule." Symptoms below the neck (chest cold, bronchial infection, body ache) require time off, while symptoms above the neck (runny nose, stuffiness, sneezing) don't pose a risk to runners continuing workouts.
There's another study by Dr Weidner who took two groups of 30 runners each and inoculated them with the common cold. One group ran 30 to 40 minutes every day for a week. The other group was sedentary. According to Weidner, "the two groups didn't differ in the length or severity of their colds."
So if your sick above the neck get out there and attack it and sweat it out!