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?
You pay them for another one, I guess. This to me reads like "if you lose it, we're not paying to replace it".
It's not like they just say "sorry, you're forever locked out". IIRC, their policy was you could order another key and pay for it with funds out of your account, and there had to be no logins on your account between the time you bought a replacement key and the time you used the new key to log in. That's pretty different from "sorry thanks for your funds".
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.
Common sense suggests to me that "inactive" means you aren't logging in, "cancel account" means you can't log in anymore, and "termination of account" means roughly the same thing.