The spread between Mt. Gox and the other exchanges is around 22% today. This may be an all-time high. We've seen 20% a few times, but above 20% has been rare. The spread has been climbing slowly for weeks. (Is this charted somewhere? "bitcoincharts.com" doesn't chart it, but they're run by Mt. Gox.)
It's a slow day. This isn't due to volatility. This is purely the penalty for using Mt. Gox.
That number is getting into the range where a run on Mt. Gox is likely.
A disconnect between markets is hardly surprising, given the delays in JPY withdrawals (previously the only fast withdrawal route) means arbitrage is going to be far less effective at making Gox track other markets. (I wonder if the JPY withdrawal delays are in turn caused by an increasing number of arbitrageurs making use of this route.)
roy