It might not happen often, but WHEN it happens that caps lock is enabled, a very visible warning would be greatly appreciated. Because master passwords tend to be long, it is extremely frustrating to re-type them in such a case.
Or maybe be innovative and offer an invert-case function? Facebook once did similarly, as per correspeonding feature request for the browser extension:
It would be helpful with input to all password/hidden text fields, not just the one for master password.