From an Electrical Engineering standpoint, they are 2 different specs, with different voltages, impedances, etc.
http://www.rane.com/note149.htmlSo what does this mean? Maybe something, and maybe nothing. Some people have made simple XLR to RCA cables, plugged them in and it works. I think it's mostly a question of how fussy your deck is.
Search around... this has been talked about before, with a particular thread started by -Q-, probably under the cable forum. No need to review it all again here.