The (somewhat) new System Sensor i4 series.

They had these on their site for a while, then took them down, and now brought them back online.

They’re available in 2-wire or 4-wire configurations. An i4 interface module is required to make them operate.

I think that these are essentially i3’s with built in carbon monoxide detectors and a slightly different exterior design. Here’s a video from SS at ISC West showing a demo of it. As you can hear, its piezo sounds somewhat like a SpectrAlert Classic’s.