Yes, it's quite normal, however, the amount of leakage is usually directly related to the amount of straining you are doing when you move your bowels. What's happening is when you strain, the balloon inside your bladder (that's on the other end of the foley catheter) is moving slightly out of position, which allows some of the urine to flow through the urethra as opposed to through the catheter. In my case (see my thread "Greg's RALP" for my day-by-day post-surgery report) this also led to a burning sensation as the urine leaked out during a bowel movement, which, although not unusual, made for some very discomforting days.
What you need to do is load up on fiber, like prunes, so that you keep your bowel movement straining to a minimum. Your doctor may have also advised you to take stool softeners each day - this is extremely important, as it is very easy to strain during a bowel movement, and the faster you go through it the better. Good luck.