I don't know about wisconsin, but i live in indiana and i can only say what i have seen. i was at a kegger when i was 18 and we got busted. i was sober (i'd only been there like 10 minutes) so my friends and i took breathalizers and were let go. the one guy there that was over 21 - a brother of one of the other guys there - did get arrested tho. not sure exactly what the charge was but i think it was contributing to the delinquency of minors or some such stuff. he didn't buy the booze and no one could prove he did - i don't even know who bought it. but yeah, i'd be careful where you drink now that you're 21. i'm 25 now and if there are minors around i leave because of that experience. don't know if that helps, but hope it does - watch who you hang out with basically. And besides, i think people under 21 tend to act a fool when they're drunk more often and it seems to me like parties are more likely to get busted when they're around so i just don't go.
