I'm not sure if good idea from the point of view of merchants. What looks better:
But this awesome pen for only 100 mBTC!!!
But this awesome pen for only 0.1 BTC!!!
The second price looks like a better deal
First of all, I'll keep my Buts and Pens separate. Secondly, 0.1BTC looks better. You'd get more sales with a price like that.
I have to keep catching myself... I think, "Oh yeah, I'd pay 5 BTC for that!" Then, "Oh wait, that's $40 now..." There's definitely a psychological sales advantage to being able to show lower numbers.
I think I would avoid the term mBTC for now, and just stick with support for more than two decimal places. Reason is, confusion for new people coming on to the scene.