Today a user commented on an Area51 post of mine about the 10 posts (questions + answers) required to fulfill a commitment. He got the Beta badge on Poker and his Area 51 profile shows the commitment fulfilled, but he's only made 8 posts on that site. Have the requirements changed? If not, is it perhaps counting deleted posts or something?
You should count all non-CW (non-community wiki) posts including questions and answers on both of the main and meta sites (including deleted once). However it's not always achieved by posting exact 10 posts as mentioned here, since the exact logic of achieving commitment fulfilment is publicly not shared not, however it's most likely between 8-10 posts.
In my case I had 7 answers & 1 question on the main site and 1 answer on meta site with rep of 300 (9 posts in total, not 10). This was enough of achieving fulfilled commitment for. So most likely participating during private beta could make the difference.
Here are some hints found in Area51 FAQ section in terms of fulfilling commitment:
Once you've fulfilled your commitment on a new site by asking and answering a few questions, you'll be able to commit to another site. If you are very active, you may be able to re-use your commitment as soon as the second day of public beta. If you aren't active on the site, you won't be able to re-use the commitment until the end of the public beta, or until six months after the beta began.