My proposal just hit beta, and people started pouring in. I expected some badge on area51, but I got nothing. A bit anticlimatic, or maybe something will happen later on ?
There is no badge1 for getting a site into beta, but there is one you should get for completing beta:
Badges aren't awarded automatically: assuming you meet the requirements, you should receive it within 24 hours.
Note 1: There's is, however, the Revolution badge (and its silver/bronze equivalents), which was to have a proposal with 100 followers or committers. Since you need 200 committers to get into beta, you should've gotten it long ago.
A bucket load of reputation that won't go away should be a good enough reward;-) Not to mention the inevitable badges that will result
Aside from that, there are a few opportunities for rewards, but the second person to join the project gets just as much as the original person, including the Founder badge, among others.