Andrew Clinick gave a talk today on the future of the Windows Phone platform, specifically on 'Mango', going into more detail than the keynote (we'll have a video up later).
During the sensor discussion, it was mentioned that current Windows Phones won't have access to the Gyroscope API since they evidently don't have the hardware to support it, in turn, only "new" devices with 'Mango' on board will have this feature. Of course, for all we know there may be one or two devices out there with gyroscope access on board already, but it doesn't seem to be the case. Either way, it does raise a conudrum of how older phones will deal with newer software that uses this feature.
Update: We confirmed with Clinick after the talk: current devices do not have a gyro, so only new Mango devices will have this. So what about software and devs? Basically with a combo of the accelerometer and other APIs for hardware, you can simulate, to a lesser degree, the gyroscope behavoir. So you can run the apps, they just won't be as awesome as devices with gyros. Fair enough.
Update 2: Per Microsoft's Istvan Cseri, the gyroscope sensor is optional, up to the OEMs