It's amazing how regularly the question of Zinfandel's ageability comes up. It's also amazing how the issue always has the same resolution: most zins don't improve with age, but a few notable exceptions might.
In my experience, some Ridge zins improve, if you accept that "improve" means they drink more pleasurably--though a bit differently--to my taste. The '92 Pagani LP that sfiorare described is a good example. In 1995 it drank very similarly to a young Port--big, bruising tannin, huge fruit, noticable alcohol. By 1998 it had mellowed quite a bit, more silky in the mouth, alcohol not so obvious. After reading sfiorare's note, I'm sorry I don't still have a couple of bottles.
I think my all-time favorite zin was an '84 Ridge York Creek, drunk in '95. It wasn't very zin-ish, but what it lost in bramble and berry power it made up for in enjoyable texture and finesse. We drank it at the same dinner as an '86 Beausejour-Duffau, and I believe everyone preferred the zin (they sometimes do become a bit Bordeaux-like when they're older).
In any event, I think all experienced wine drinkers at some point make the decision about whether they prefer their wines younger or older, and they do what suits them best. Different strokes, and all that.