What does MtGox's terms of service say about yubikeys?
Mt. Gox shall not replace any key lost by a Member.
So what happens if you lose one?
We also reserve the right to cancel unconfirmed Accounts or Accounts that have been inactive for a period of 6 months or more, or to modify or discontinue our Site or Platform. Members agree that Mt. Gox will not be liable to them or to any third party for termination of their Account or access to the Site.
The fact is MtGox has no clear policy on accounts with yubikeys. The terms of service do not seem to clarify the meaning of "inactive", "cancel account", or "termination of account". If you lose your yubikey, MtGox might be nice and violate their own terms of service, or they might repeat "We apologize for any inconvenience caused" until your six months are up.
If you do have an account with a yubikey take your money someplace with competitive service policies in place as fast a possible, lest you get robbed like me.