I signed up to post this info because I've been looking for this exact information:
I found the pearlescent revolver off of Crawmerax just now. No big deal you say? Here's the details I wish *I* had, as the find was purely accidental.
I basically gave up on deliberately finding a pearlescent...apparently, they're so rare, they may as well not exist in the game. So, having said that, I was harvesting weapon proficiency off of Craw and his buddies and out popped a goofy colored item. On the ground, it's definitely hard to miss with all the blues and greens - it sticks out as bluer than the greens and greener than the blues. Cyan definitely. Can't miss it laying there in the piles of loot.
Playthrough 1 (!)
COM: catalyst, garbage and +2 rare item find (for the off chance for a penetrator, lol)
Since I didn't care about loot, I phasewalked for efficent proficiency harvesting, busting the myth completely that phasewalking nerfs Craw's drops for Lilith - which I've read here and there.
I killed crawmerax about 10 times and gave up on the pearlescents and instead farmed him for proficiency EXP, with drops being ancillary.
I estimate - in total - that I killed Craw more than 20 times but less than 30.