When does MtGox lock accounts? When you sign up at MtGox, they say: please fill out the following to avoid your account getting locked.  They're referring to the member verification process (using govt ID and utility bill).

But under what circumstances will they lock my account if I cannot provide verification docs at the moment?  And what does "locked" prevent me from doing?  Does it prevent me from, for example, transferring BTC to another exchange or wallet?
I recently got mine locked because my YubiKey broke.

When you want to disable the YubiKey for your account, your account will be frozen for two weeks. I had my account information verified and still got locked.