Then we should refer to the mBTC as THE BTC coin and the wallet should list the mBTC as the coins contained. Meaning if you have 0.5 BTC you see 500 in your wallet.
But this is missing my point. Do not think about this issue from a pure technical or mathematical solution. Yes, I am aware we can go down to 8 digits.
Think about it from a Bitcoin mass-adoption perspective. Businesses, merchants, "regular" buyers and sellers are not used in calculating and dealing with 0.00xxx prices. I strongly believe that it would help Bitcoin adoption if most of the transactions will be priced in that sweet spot range of 10-2-104.
If you are using the standard client, you can already set it to display mBTC or even μBTC. So just set it in the way that you are the most comfortable with.
Solved!