The full release notes for this release can be seen here: http://arcengames.com/mediawiki/index.php?title=Tidalis_-_0.409_Beta_Release
It's been another long stretch with few beta releases, and so again of course this is absolutely feature-packed. We keep hitting these large features that require a lot of behind-the-scenes work and testing (first multi-well play, now VS mode and the AI), and which tend to break a lot of the game until we get them fully back the way we want.
There's a possibility that our upcoming work to get network play added to the game will have a similar effect, but a lot of the preliminary structuring of the code for that has already been done, which hopefully means that the network stuff will be more isolated than the last two big things. Of course, I thought the same thing about the multi-well and VS code, so I guess that's how it goes.
On the bright side, other than network play there aren't any other big features with that sort of huge scope and touch-everything-in-the-game nature. Other than getting network play in place, we're down to adding more special blocks, more items, more modes, more themes, more levels, more story, and then a bunch of fairly modular features like high scoreboards, achievements, gamepad support, etc.
There's still plenty to do, but we're looking good for the July 1 "done with everything" date. Whether or not the game will be fully feature-complete on June 1 or not is a huge question mark programming-wise, but that's largely because we keep doing so much polish as we go, rather than leaving it all to the end. That seems to be for the best, but we'll see how it affects our pre-release milestone dates as we get closer to them.