There is no mirror lock-up mode per se on the Nikon D90; instead, there is a feature called “exposure delay” (menu item d10, under custom settings) — it lifts the mirror up a second or so before the exposure is taken. (The D80 also had this, but the delay was apparently shorter.) Mirror lock-up is used to reduce vibration from mirror slap. In a single-lens reflex (SLR) camera, the mirror flips up and down with each exposure; this can jiggle the camera on long exposures. So night photographers and astrophotographers like having mirror lock-up available. On the other hand, on really long exposures of several minutes, a second’s worth of vibration won’t be noticeable.