The crankshaft pulley has a rubber insert between the crankshaft and the pulley that drives the belt, to allow the assembly (commonly known as a harmonic balancer) to absorb vibration. It has been reported that the rubber tends to separate. When that happens, the pulley falls apart, the belt falls off and you loose cooling (water pump), alternator, power steering and AC.  Not a good thing to have happen anywhere, let alone on the open road.  

I've never had one fail.  The one on my 91 went 140K (still going strong when the subsequent owner had an oil loss and totaled the engine).  The one on my 94 has 110K on it and looks fine.  I think the most important thing you can do is keep the engine from leaking oil onto the pulley - oil attacks the rubber in the joint between the two parts of the pulley.  Make sure the front main engine seal doesn't leak.  If it does, have it fixed, pronto.

You might want to replace the pulley/harmonic balancer every 100k miles or so, at a cost of $100 to $200, but I think that if you keep the engine oil-tight, you won't have to worry about it.  

The single known warning sign is a squealing sound, as of a belt slipping, particularly at startup; if you hear this noise, get the pulley looked at very soon, if not immediately.  The sound is caused by the outer part of the pulley spinning on the failed rubber joint, and total failure is imminent.  The crank pulley should be inspected yearly at a minimum, and any time the fender liner is removed.  Here's a way to mark it for easy inspection (no return to this page).