I'd try your best to make it to the pre/post interview social events. If it's a pre-interview event, it alllows for you to ask some questions and possibly get some ammo for interviews the next day. It's also a good time for you to see whether you "fit-in" with residents from a particular program. So don't just sit on the wall and talk with other applicants. Make the rounds and say "what's up" to the residents. Most orthopods are fairly out-going. I think it says a lot when an applicant takes the initiative and tries to get to know the program that he/she came so far to see. Also it's nice to bring your significant others, because some time they can take a step back and give you some in-sight on whether you REALLY fit in or not. Some times you get caught up in the moment as an applicant and can't really discern whether you fit in or not. Your significant other may be the fly on the wall that realizes that you're way too cool for a program or vice versa (truth hurts some time
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).
At Case, if you don't make it, no one really takes note. For us, I think it mainly functions as a tool for the applicant to get to know US better. Relaxed, no pimping, just a bunch of guys/girls hanging out and having some beers. From our standpoint the social won't get you into the program, but it could get you out of the running. We had an applicant last year that was trying to grind with some of our interns wives. Poor form.
As for dress, it doesn't hurt to be on the safe side and drop an email to the coordinator prior to interview time. If it was at a bar/sports-bar, then I'd wear bar/sport-bar type clothes (i.e. jeans, sweats etc.). Some places have a more formal sit-down thing at a fancier place. Shirt and tie may actually be in order for these places.