I get your point about making it user friendly - although to me it makes little difference whether prices are displayed in BTC or mBTC. Consistency could be an issue here though. It certainly becomes less user friendly if prices are displayed in BTC in one place but mBTC elsewhere. Maybe best to stick to BTC and learn to deal with leading 0s?
Or switch to mBTC and have them become familiar with SI units from the get-go? Surely, people wil ask what mBTC stands for or why the m is small, at which point a discussion along the lines of "it's like megs and gigs".
mBTC is only the start. If Bitcoin gains wide adoption, there's no reason to think that uBTC won't eventually become the norm.