Dice do have imperfections (depending on how expensive ones you get), but would they not still have enough entropy such that a private key generated by them could be considered really safe? I am thinking of paper wallets storing lots of coins, potentially.
It would short circuit the problems with the implementations of random number generators on computers (the recent Android bug, and the Debian bug between 2006 and 2008)
If a die is missing a side one would notice, so they are a lot easier to inspect for big errors than computer code.
So, how much would imperfections in dice lower the entropy? Could you just calculate with a lower base, e.g. 5.765*N rather than 6*N with a perfect die (5.765 then being a measurement of imperfection of a specific die).
Edit: maybe rather 6**(N-x), where x are the extra occurrences of one number.