Current prices:
$120 = 1 BTC = 1000 mBTC
100 mBTC = $12
one 7 buck tee = 58 mBTC = 0.058 BTC
"I'd like a t-shirt for point zero five eight bitcoins please"
OR
"I'd like a t-shirt for fifty-eight milliBitcoins"
Take your pick.
Neither sounds great to me because milliBitcoins is a mouthful. Let's start another "how can we make mBTC into a single syllable" thread, find a great shortname for mBTC, and then switch to mBTC (until its time to drop down another 10^3 in a year ).