For instance here: http://bitcoinwatch.com/ one exchange lists the BTC-USD rate at 133, while the other one lists at 122.
Because MtGox is delaying USD withdrawals for several weeks so whoever wants to withdraw funds quickly has to do it buying Bitcoin and withdrawing in Bitcoin. This creates a difference in prices. The question is why people still keep depositing to MtGox? May be because MtGox still has the biggest trade volume in the world, or may be they are old deposits