Microsoft is now showing prototypes of tablets with auxiliary displays. The idea is that there's some stuff (at the very least, “your next appointment”) that you want to see all the time. The exact definition of what this stuff might be is extremely fuzzy -- for instance, I've heard people say “Email” but for laptops that almost certainly involves activating the power-hungry WiFi radio, and I've heard people say “Driving instructions” which would presumably require both an active display and probably some kind of connection to a GPS device. I've also heard people say “MP3 playback,” and a moment later, “Video playback,” and a moment later, “And I want to be able to take a quick note. With my pen.” And while we're at it, I'd like to be able to jot down a quick algorithm and visualize its behavior over time...
Aux displays themselves will be optional components, presumably on high-end laptops. Therefore, developers will not be able to count on the presence of an aux display. Meanwhile, everyone who buys a high-end laptop has an auxiliary display device they already are committed to carry -- their cellphone. I desperately hope that Microsoft's aux display SDK supports “disconnected aux displays,” i.e., Bluetooth-connected cellphones, media players, etc.
I desperately hope for that because the transformation of a powered-down laptop from a brick to a gently-sleeping giant is a very important capability. There are all sorts of scenarios where a low-power-consumption computation can trigger more interesting behavior. For instance, if I have a connected GPS, I can wake up every minute or so and confirm that I'm within, say, a mile of my route and, if not, automatically fire up the computer into full-power for the power-consuming tasks of map display and rerouting. Or, I can benignly “war-drive” -- waking up and trying to sync my email when I'm in a WiFi lilypad.
These sorts of capabilities are much more interesting and valuable than 240 x 170 pixels of auxiliary display space.