Its so funny that you brought this up, because I was actually wondering the same thing. I've been working with a lot of japanese crochet patterns, and 3/0 etc. is how they display the hook sizes. I finally tracked down one of my friends who went to college in japan to see if he could help me translate, and he found me a site that explained the conversions! I'll put it all in here for you...
2/0-3/0 2.0-2.3mm US B/1
4/0-6/0 2.5-3.5mm US C/2 D/3 E/4
7/0 4.0mm US G/6
7.5/0-9/0 4.5-5.5mm US 7 H/8 I/9
8/0-10/0 5.5-6.0mm US I/9 J/10
I hope that makes a little bit of sense...