Baseball Prospectus published its RBI Opportunities by Batter Report for 2004. Some notes: R1, R2, and R3 indicate how many total runners were on 1st, 2nd, and 3rd base respectively. PA is plate appearances, BI indicates how many runners the batter knocked in, and the final number gives a percentage of opportunities converted into RBIs (BI divided by TOT). Note that the BI number doesn't match up with each player's final RBI total--that's because it only counts the runners on base who were knocked in, and not the batter (simply adding the player's HRs on the season to the BI number will give you his actual RBI total).
Here's a shock--ML RBI leader Miguel Tejada had the most opportunities with runners on base in the majors. Tejada easily saw the most runners on 3rd (120) in the majors, which no doubt helped him considerably. Still, Tejada had one of the better conversion percentages (.212) in baseball, so he was doing plenty to help himself.
Compare the Opportunities by Batter Report to the list of RBI leaders and you'll see a pretty obvious correlation. You'll also see that the guys at the top of the report are the 3-4 spot hitters (not that this is any revelation). The report helps to illustrate why RBIs have more to do with one's place in the order than they do with an extra level of skill. If you compare the percentages, you'll see that the guys at the top of the list aren't really doing anything better than the guys in the middle of the pack--the guys at the top just have more chances.