+1 for this

The word "Bitcoin" has too much momentum to subvert it now, and any attempt to do so would only further confuse the masses, which are already pretty confused by this whole Bitcoin thing as it stands.

The base unit of the Bitcoin currency is the bitcoin. As we're all scientifically minded folks here, it makes sense to use the SI prefixes to denote smaller (and larger) quantities of bitcoins.

The abbreviation of the bitcoin unit is BTC. It really should have been XBC, if we were to follow the rules of the ISO regarding currency codes, but it's not, and again, we're dealing with momentum. You can't easily change it now, nor should you, for the aforementioned reason.

Thus, the logical conclusion is that quantities of bitcoins may be expressed in units of millibitcoins, and the abbreviation of the millibitcoin is mBTC.

In casual parlance, I wouldn't expect anyone to utter the phonemes "em bee tee cee." I'd expect people to say "mils," "millies," or "millibits." I'd expect "millibitcoins" will be considered too formal for casual utterance.

I personally will probably say "mill," dropping the 's' even in the plural. "Are you kidding me?! 42 mill for a beer?!"

Fork the repository, commit your change, and submit a pull request.

This would require the developers okay. You'd need to be able to convince them. I would say that you would also have to convince all users but unfortunately that is not the case anymore seeing as to how easily Gavin was able to make changes to the client and simply tell everyone you either get with the new or stay behind. Of course I'm referring to the exclusion of certain transactions from the blockchain. Now, whether you thought that the change to the new client version was good or not does not matter the point is that a change was easily made by elite members. So there's your answer convince them that your proposed change has positive outcome and the rest of us that use the Bitcoin-qt client will have no choice but to accept it.

1 bitcarat = 1 carat bitcoin = 1/100 BTC, still gives a feeling of expensive

First of all, I'm not familiar with Github, second, I'm not qualified to make the programming or configuration change. For those reasons someone else will have to do it.

EDIT: I don't mean that rudely at all, I'm just saying that I can't do it.

However, what you prefer to call it in casual conversation is another discussion. There you have my vote for "millibit", short for "millibitcoin".

Often orders for small items in my store are priced in the mBTC range.  I have orders down to 16 mBTC SHIPPED for a set of QR stickers for example. 

My contribution.