There are delays trading between dollars and bitcoins so as the different exchanges vary, people can't easily arbitrage it and close the gap.

Bitcoin is more expensive on MtGox because there are severe delays when getting dollars out

Because the price is based on what people are offering them for/buying them for on the different exchanges.

Because different exchanges make it easier/harder to work with different currencies, different people use them.

Because they are sold in different markets. Its just like a fish or vegetable market, supply and demand at the market will set the price. The difference with traditional currency markets is those markets are very deep and competitive so the slightest discrepancy in price between markets is an arbitrage opportunity for someone which then pushes the price in line with other markets.

The more liquidity there is on the BitCoin markets, the less the price will differ. BitCoin just doesn't have that sort of liquidity or market depth yet.

Mad panic to buy means people aren't price sensitive by a few %. "$930 for $900 of bitcoins? Sure no problem, it'll be $950 tomorrow" kinda thing.

I believe it has to do with the likelihood that you'll be able to sell? Some exchanges have a lot more users.