The P100 is not JUST a "replacement" for the P90-Significant changes have occurred, first and foremost this:
YouTube - Nikon P100 Review Sample
1080p video and stereo sound.
ISO got halved from 6400 to 3200 but ONLY because (presumably) the P100 NOW has a CMOS sensor instead of a CCD.
10.3 MP replaces the P90's 12.1 MP but this is not a demotion. A 10.3 file will more than comfortably produce a 20x30 inch print, beyond which, most of us have no need of, myself included.
26x zoom instead of 24x
Shutter lag has been noticably , if not greatly, improved. You will have to check this out yourself, however.
Complete tech here:
COOLPIX P100 from Nikon
If you want to get the P100,Cool! BUT you could probably pick up a P90 for a sonf if you looked around. Either one of these cameras will do you proud, I'm sure!