It's pretty standard, if you ever played online poker you'd have encountered it too. They're very lax when it comes to depositing, but when it comes to withdrawing you first have to supply some proof about your identity before you can withdraw or transfer off a site.
I never had a problem withdrawing bitcoins or mtgoxUSD codes in 2 years. Now it happens. I talked to neofutur for a bit on #mtgox and other than having used tor or having logged in from different countries (!?!) the only reason he could speculate about was that aml shit was triggered.
I didn't withdraw more than $4000 dollars this year. Maybe if you add bitcoins (I reduced my trading funds on gox substantially by withdrawing bitcoins), but I can't see how withdrawing bitcoins would trigger any money laundering tresholds. I never withdrew dollars from gox to any bank account.
So I'm still dumbstruck. I submitted a support ticket, let's see what'll happen.
Sux to have funds frozen.