For the GM, I'd suggest he read several of the books, or else you take a crack at GM'ing. If you have experience GM'ing any game, or your present GM is very good with multiple RPGs, things should be fine. There's also a non-spoilery thread that sums up the Dresdenverse, but it's not in-depth enough for a GM (I'm hoping someone remembers the link so you can see it regardless.) Personally, I'd stress the following:
*Why no one knows about supernatural.
*White Council- its structure, positions, rules (and members if there's time)
**Harry Dresden- for the lolz, and to quell any general questions
*Vampire Courts- Explain the 3 main Courts (mention Jade or if you're feeling super-spoilery you can tell them how many Vampire Courts there are in total, including the currently undescribed minor ones)
*True Believers- How faith magic works, examples like Shiro Yoshimo.
*The Fae- especially the 3 divisions, Knights, iron/steel weakness, and deal-making
*The Unseelie Accords (this will prevent power players from killing off a scourge nilly willy)- LETTER of the law, Archive (if there's time)
*Were-Form- someone will want it.
*The WAR (if the game's time frame applies)
*Supplements (bring up ONLY IF NECESSARY to gamplay): Denarians, The Circle, specific names
*For the record, I'm definitely trying some CoC once the BAT gets here. Lots of Mental Rolls, I'd expect. It'd be a lot of saving rolls, repeatedly- I'd either invest in characters that have some kind of protection, or high mental/social tracks.