I have taken Magnesium w/o taken probiotics, and it really didn't have too much of an affect on me... though it did help a little. For me, probiotics are defintely what helps (for example, yogurt is wonderful as a tool to keep me regular). However, you raise a REALLY good point -- for many people, it could affect their bowel movements... positively or negatively...
I have seen it added to many probiotics, but I assume that it is just a trace ingredient. But, as I said, some people are very sensative to even trace ingredients of certain foods, enzymes, vit's/min's.
-------------------- "The most wasted of all days is one without laughter." -- e e cummings