At http://stackexchange.com/newsletters there's a list of newsletters one can subscribe to. Since Area51 is not exactly the same format as the other sites within the SE network, it doesn't have a newsletter by default.
However, I'm registered at this site, and I'd like a newsletter since I don't visit as frequently as I do at most other sites.
As per the comments on my initial question (thanks to Benjol for mind-reading skills), this newsletter could contain:
- new proposals
- proposals that are nearing end of commitment phase
- new sites in beta
- new graduates out of beta
- betas that "need work" in some area. For example, (at the time I'm writing this), Travel needs work on questions/day and visits/day.