A bunch of third party stuff works already, as Securifi sold only a few different sensors to backers in the beginning most of what people are working with are third party devices. That said, not all types of devices work, they need to be implemented. The best way to find out if they work if you have one is to pair and test it. If you don't own one, ask on the forums here (like you did for the motion sensor) after checking the wiki with the list (in the stickies under tested devices) and search the site. As j8048188 mentioned Iris tends to have something proprietary built into their protocol layers. I don't know if it is all their devices or only certain ones and I am not sure if Securifi can add those devices without the manufacturer input / assistance or if they plan too.
Edit: @j8048188, check out the quirky tripper. Same price point (but in packages of two @ $40) and confirmed by several members of the forum to be working fully. They are carried on the quirky / wink websites (though out of stock on both of those last I checked), amazon (with free shipping) and I imagine eventually on Home Depot's website and at their brick and mortars. I am currently awaiting the arrival of a few of mine.
I checked around and they are the cheapest z wave / zigbee motion sensor I could find AND they apparently work with A+ out of the box. I'd love to see something cheaper for just a basic sensor like this but who knows if that'll come around.