Actually, body based IS systems use an internal gyroscope to sense camera movement, not an optical system. If you have a 100MM lens on the camera and the IS system thinks it's a 50MM lens, it won't compensate for shake enough making the IS useless.
Originally Posted by SONYNUT
On the other hand, if you have wider FOV lens than a 50MM and it's still set at 50MM, it'll compensate too much, adding blurring to a picture that wouldn't otherwise have it.
Zuiko 14-54 F2.8-3.5 MkI
Zuiko 70-300 F4.0-5.6
Konica Hexanon 52mm F1.8