Thanks for this. I've been seriously needing to look into this and you have lit a fire under my ass to do so now. I'm wondering about existing accounts though, like if I can just add my sons' current accounts, gamertags and all to this plan. If I don't find the an answer online then I'll call the 800 number and find out then post it here.
This isn't the answer but I did just find this,http://www.vgrevolution.com/2010/11/microsoft-having-wide-spread-xbox-live-billing-problems/
Ok dudes, this should answer just about all questions, http://www.xbox.com/en-US/Live/JoinLive/FamilyPackFAQ
It's a good plan and I will be getting down with it once the issues in the other link I posted are resolved. I'll have an extra account to fool around with too.