This might be a silly (read: newbish) question, but what's stopping someone from buying a few Bitcoins from BTC-E at 87.849 USD per BTC, and then selling them at MtGox for 118.2 USD per BTC to make a profit?
Nothing. And you got the prices totally wrong. It's about 87 EURO / 1 BTC on BTC-E. If there were such a huge difference, people would have exploited it already.