See http://meta.stackoverflow.com/questions/5983/should-all-linked-accounts-get-the-100-bonus
You need to earn 200 rep on any SE site, then any other site that you link to that account gets +100 rep.
This is why a lot of people start with 101 rep. Essentially the system guarantees that if you've become minimally trusted (>200 rep) on any site then you deserve to be advanced past the opening jitters on any new site.