I started getting stitch for no good reason just after Christmas. I asked a ex-professional elite runner about this at one of the golden bond training days for the London Marathon in Feb and he and his coach both agreed on the answer.
Apparently if you blow out as if to blow out candles on a cake, hold it, and try to blow out anything else you have left in your lungs, THIS WORKS. I tried this on my next few runs and although I found it difficult to hold it once blown out, I could feel it pulling on my lower abs and strangely, the stitch eased after doing this a few times. I think the ideal thing to do is to stop and do this but I found that if I concentrate on exhaling more than normal on each breath during normal running, this also seems to work after a little time. You do end up sounding like you're exhausted though!
This method might work because you're concentrating on your breathing, but as long as it works, do we care?!
Give it a go and good luck!