1) It’s already available:
http://rayshobby.net/cart/osdc
We will make it available on opensprinkler.com as well (still finishing up the product page, but should be done within a few days).
2) The main benefit of the DC version is that the power adapter (9VDC) is easy to source and the same adapter works universally everywhere. If you’ve seen 24VAC transformers — they are heavy, bulky, expensive, and do not work universally (e.g. output voltage is proportional to the mains voltage). This is the main reason for the design of the DC version. For US customers, this is probably not a big deal, but if you ever need to replace the transformer, it might still be an advantage.
I consider 9VDC to be safer than 24VAC — the peak voltage of a typical 24VAC transformer can reach as high as 40V, which is somewhat dangerous. If you accidentally shorted the wire to something it can result in big damage. In contrast, 9V is a lot safer.
Also, in the past we’ve received several requests to make OpenSprinkler compatible with DC solenoids, and we ended up having to customize the controller for these customers by replacing triacs with MOSFETs. With the DC version the same controller can now work with both AC solenoids as well as DC solenoids. That’s a another advantage.
Going a little more technical: the MOSFETs used on the DC version can sink as much as 3 amp continuous and 30 amp impulse current. So it’s a lot more capable in terms of handling shorted solenoids and so on.