On down-level DCs (2000 and 2003) the "Unlock account" checkbox used to be disabled if the account was not locked out, since if the account is not locked out there would be no reason to select that checkbox.
But in Longhorn, RODCs were introduced so now the "Unlock account" checkbox is always enabled now regardless of whether the account is actually locked or not. (Unless the user who is viewing the dialog box doesn't have permissions to write to the lockoutTime attribute of the account. Then the checkbox would be disabled.)
Resetting lockoutTime to 0 unlocks an account. It's really just writing to an attribute on the user account is all that the checkbox does.
Setting lockoutTime to 0 in turn triggers urgent replication and resets badPwdCount to 0 as well.
If the user's lockoutTime is already 0, then setting it to 0 will reset badPwdCount to 0 and if badPwdCount was already 0 then it has no effect.