The price isn't being set arbitrarily. It was $17.5 the second before the big sale, and that's what it's being rolled back to.
however, that will not correctly dictate the state of the market when the site comes back up, meaning its an arbitrary number at the time. If your buy and sell orders are based on that, when say...tradehill or some other exchange is significantly higher or lower, it stands to reason that you will lose or gain accordingly based on the state of the market as a whole. Its basically a fix, since everyone now knows that mtgox will be at 17.50 at around 1AM GMT.