why use mBTC?
Due to the high valuation of the basic Bitcoin unit, many everyday items and values can only be priced as fractions (like 0.031 BTC for a hamburger).
Using the mBTC unit allows for simple, easy-to-understand whole numbers instead (like 31 mBits for the same burger).
The mBTC unit is also somewhat forward looking, because it allows for a significant further rise in Bitcoin valuation without needing to adjust units.

how do you say "mBTC"?
Technically, "mBTC" is an abbreviation for the word "millibitcoin", but you won't find that word in common conversation.
Instead, "em-bit" is a lot shorter and easier to say aloud.
If you need a more formal word, either "millibit" or "millicoin" will do as well.

how do I change my wallet to display values in mBTC?
Here are instructions for several common wallets:

If you're using Bitcoin-Qt, go to Settings > Options and choose the Display tab.  Under "Unit to show amounts in:" select mBTC and hit OK.

If you're using Blockchain.info's My Wallet service, go to Account Settings > General.  Under Bitcoin Display select MilliBit (mBTC).

If you're using Electrum, go to File > Preferences and choose the Wallet tab.  Under "Base Unit" select mBTC and hit OK, then restart Electrum.

If you're using Multibit or Bitcoin Spinner, please be patient--mBTC support will be added soon.

If you're using any other wallet program or service, please contact the wallet provider for more info about mBTC support.