It is a larger mirror. Focal plane shutters do also cause vibration. On the Pentax 67 shutter vibration was worse then vibration fro the mirror. Why do you think that IS does not compensate for vibration from mirror?
The effects of mirror vibration when using IS for hand-held shots is an interesting subject worthy of some careful testing. It certainly seems reasonable to suppose that the floating element in an IS lens is not able to react quickly enough to counter the relatively high frequency vibration of mirror slap, and probably the same for the anti-shake sensor.
However, the question is, do the conditions of holding a camera firmly with both hands and pressing it against one's cheek, have the effect of providing additional dampening of mirror slap so that it ceases to be a problem (or at least becomes less of a problem)? Without having done specific testing of this issue, my guess would be a yes.
If the answer is 'no', then that has serious implications for the value of IS in wide angle lenses. It would partly explain why Canon has not provided IS in a number of its wide angle primes and zooms , and it would suggest that the claimed additional benefit of the anti-shake sensor in the A900, in respect of these wide angle lenses which sometimes don't have IS in the Canon equivalent, is misleading.
My own tests have confirmed that the effects of mirror vibration when camera is on a tripod are most pronounced around 1/30th of a second. They get gradually less as one moves away from that shutter speed, in either direction, so by 1/60th and 1/15th exposure, loss of sharpness due to mirror slap is (or can be) insignificant.
Now it's clear that this range of shutter speeds, 1/15th to 1/60th, is the range which is most likely to be used with wide angle lenses in poor light. It's the range which, without the benefits of IS, one could not expect tack sharp results.
So basically we're caught between a rock and a hard place. If lighting is good so we can use a shutter speed faster than 1/60th, we don't need IS (with wide angle lenses). If lighting is poor and requires a shutter speed between 1/15th and 1/60th, IS won't help much because of mirror slap. If lighting is so poor that a shutter speed slower than 1/15th is required (1/10th & 1/6th sec etc), then IS will help, but you really can't expect a truly sharp image at such slow speeds, although one might get results acceptably sharp for small prints.
Edit: Some more thoughts on this issue. The Live View feature on Canon's latest DSLRs gets around this problem nicely. If it's true that mirror vibration is not sufficiently dampened when camera is hand-held at shutter speeds from 1/15th to 1/60th, then using Live View solves the problem. The mirror is up and the view is stabilised if the lens has IS. With Live View in all its latest cameras, there is now a good reason for Canon to provide IS in all its wide angle lenses. In this context, the lack of a Live View in the A900 can be seen as a major disadvantage (unless it's true that pressing a camera to one's cheek dampens mirror vibration).