Get it here.
This beta has some syncing fixes and some other unrelated upgrades. The old simultaneous-use warning that was added in 5.1 is gone, in favor of watching the file system for changes and updating automatically. There's a fix for tabs, and for time zone switches in the calendar. The recording devices in the pref pane should update more consistently now, and page setup information is now included with files dragged out. So perhaps there's something there for everyone, but let me know of any problems.
UPDATE: There was a regression with rich text entries with attachments in beta 8. I've updated the link above to point to a fixed release, 5.2b8.1. If you downloaded 5.2b8, you should see it offer the update.