@adamz: the 1/5000 is gained by having a stripe of light moving cross the sensor. actually the whole exposure process from top to botton of the sensor takes 1/125 of a second or so -- afaik. that's where the flash synch time comes from, cause the flash fires only briefly. with an exposure time of 1/5000 you would have a single narrow stripe on the sensor exposed by the flash. so even if in any given spot the sensor ist only exposed for 1/5000, the exposure of the bottom pixel and the top pixel are separated by 1/125 sec.

@mb: i was wondering, where those 4 shots/sec come from. of course the mirror flip takes its time. so if you skip any viewfinder throughput or AF metering in between shots, you don't need the flip and could speed things up, possibly. in this case the limit should be imposed either by the the readout time from the sensor or by the actual shutter mechanism (not the mirror) or by the aperture mechanism in the lens -- or something else? in video mode there are 24 pics per second read out, right? so it should be possible to develop a "speed bracketing" function. i'll wait for that.

It does close itself but D90 could not shoot any faster that 4 and a half shots per second at that is that as far as I know.

as for my knowledge it shuts completely, and where did You get the 1/125 of a second closing time - if You can adjust times like 1/5000 so it must close itself in this time

It does, I mean mirror flips several times during bracketing, and as far as I know it is the same on all SLR cameras, unless you lock the mirror.
On a D90 minimum is something around 1/222 between shots actually.

When doing bracketing on a D90, does the mirror flip several times or just once for all three exposures? How is it on other cameras?

Of course it would be cool having three pics taken only 1/125 of a second apart?! That is the time span the curtain need to shut completely I believe, or does the sensor readout set a limit to bracketing speed?