The Enterprise seldom had to initiate saucer-separation, but it certainly happened on more than one occasion prior to the movie. The instance that most readily comes to mind is the episode with the Echo-Papa-607 supercomputer; I seem to recall the saucer separating during one of the battles against the Borg, as well.
And yeah, I'm a geek.
Edit: For those of you questioning the perils of an exposed bridge, look no further than The Wrath of Khan. The Enterprise pummels the crap out of the Reliant, firing directly on the bridge on two occasions: on the first, the character heavily-implied to be Khan's son is killed, while on the second, Khan himself is gruesomely wounded.