I think a relaxed conversation to gauge what they expect would be the best way to start, ie. "so are where are you planning on having bubs?"
The maybe let it gently go from there, say to them "look i have LOTS of information i would love to share with you about different birth options etc, IF YOUR INTERESTED"

I think the key would be asking if they want to know rather than getting excited and bombarding them with information.. as you said.

It's so nice you want your friends to be informed and be able to make a decent choice based on their needs, you can always refer them to BB as well.. i found thats a wonderful help for people and they like the fact they can be anonymous.

For me before my first i think i would have loved more information on home births, it's not something that i had ever thought about, i had never even considered it, i thought hospital was just the 'normal' thing to do... in the end i had a home birth but not by choice and it was an amazing experience.

GL, sorry prob haven't helped at all.