Its a simple math thing, to multipy by 1000, in order to move the decimal place 3 places to the right.
I don't know why this would be difficult, but I don't know much about the code for bitcoin.
It does depend on whether this can be done in the code. It seems like a simple thing to me.
Mtgox is thinking about it, as can be seen at their graph site at bitcoinity.org/markets
where they are using the mBTC now.
The client already supports mBTC. It is in the options. MtGox does not own or operate bitcoinity. Bitcoinity did that on their own.
Individual companies are doing this on their own. Bitcoinity already does this, Mt Gox is thinking of doing this. Another company that does this already is the Bitcoin Investment Trust (BIT) via SecondMarket. They quote the BIT's NAV at 1/10th the actually Bitcoin price, updated daily once per day. So currently the NAV is ~$83/share. Makes people feel like their getting more shares of Bitcoin.