Get it here.
This is a larger release than 5.1.4, but smaller than 5.1. It's largely to support a future iPhone client, but there are some features in there for desktop users as well. For example, LiveJournal users can download more than 50 entries. Also, non-main documents are backed up. Quick Note works better on Snow Leopard as well. However, this release is already feature complete, so nothing else will be added except for bug fixes.
There are a fair amount of under-the-covers changes as well, particularly in blogging, so please try it out and let me know if anything breaks.
9/22 Update: I fixed the beta expiration date for non-US users. If you were seeing a problem, please re-download 5.2b1. There are no other changes in the fixed build.