As long as you input the actual focal length of the lens you are using, it works to the same constraints as with a native lens. IE, don't expect miracles, just use it as an aid.
As Stephen says there is a way to input the (actual, ignore crop factors) focal length - I knew how to do this on my E-3 so I hope someone will share the process used with the Pens.
My way of getting to this function is as follows: Select the IS function (there are many different ways of getting to this which in my case seem to vary day by day) then press the top exposure compensation +/- button. Use the control wheel to select the actual focal length, then OK out of it. Note that in some instances, eg 25mm, you have to select the nearest, but most focal lengths are covered. Also this is for the E-P2 - I don't know if it's slightly different on other Pens.
The IS feature is a physical process performed on the assembly that holds the sensor to counterract the movement of the camera, so it works on both raw and jpeg files. It is not a software correction.
Always the actual focal length. (Crop factor should be abolished as a useless and confusing custom. It serves no purpose.)
To use the IS with an adapted lens on the E-P2, navigate to IS3, press +- (exposure compensation) button on top of the camera, then use silver dial on top right back to dial in the focal length of the lens. Use the actual focal length, i.e. for the 45mm lens, dial in 45mm.
