The title pretty much says it all really! Austin has started public testing on X-Plane 8.50 Beta 1. You can download it in the usual manner. You will need to delete your 8.40 preferences files to have 8.50 work correctly (which means setting everything up again), however it seems to be quite a few nice changes.
Update: As always, as soon as I publish this, Beta 2 is now out :)