OK, here's the scoop.
I have ten old-label slabs, and all ten have some bowing.
Seven of the slabs are very thin, and the other three are very thick. The thick slabs exhibit significantly less bowing than the thin ones.
Of the seven thin slabs, the one with the least amount of bowing is also the newest (dated 6/30/2002). The one with the second least amount of bowing is the second newest (dated 8/6/2001). (Those two also haven't been in my collection all that long--maybe five years.) The one with the third least amount of bowing is--dig this--the third newest (dated 2/26/2001), but that one is only slightly less bowed than the remaining four, which are all similar (dated 7/18/2000, 9/23/2000, 1/22/2001, and 1/25/2001).
Incidentally, the thicker slabs are dated 9/14/2000, 2/3/2001, and 6/26/2002, and the two older ones have been in my collection since 2001.
I did find a couple of newer slabs that have some slight bowing; those particular slabs are of the very thin variety as well.
The bowing may very well have been present since the slabs were made; it's possible that I simply didn't notice it. At any rate, the condition does seem to be worse in the older slabs, but only in the thin ones.