Just found this site and I had to jump in on this thread.
I am also from Ohio U - alumni since 1996. One of my associate class brothers started Hottubbing at OU back in 1992 when he was Epsilon. His original theme was "Hottubbing for the Homeless", and we raised money for a local homeless shelter. As the years went on, the event grew in popularity, and I'm really glad to see that our guys continue the tradition of hosting the best Greek philanthropy on OU's campus. I'm sure he will get a kick out of the fact that other chapters are now getting in on the fun.
I almost hate to say this, but the original reason the event was started was because this guy was looking for an excuse to sit in a hottub with sorority women.
Luckily, it was so well thought-out that we were able to make it a philanthropic event rather than a "party".
Anyway, the keys to making this a successful event have already been covered. But a bit of advice - you get more "pub" if you do this during the winter (if you're in a cold climate). People check it out out of curiosity - most think you're crazy to sit in a hottub when it's cold out - and that gives you a larger audience from which to solicit donations.