You're right, of course, wear is a possibility. I hate to believe it though; hard to imagine getting decent mileage out these things if there does turn out to have been noticeable wear in so short a time. Also, neither rear is showing the same loosening leading to slight lateral play at the rims (I built two wheelsets around identical hubsets and switched out both front and rear when I noticed the slight play up front).

I had not thought of spoke tension stretching the hub; that could have something to do with it. I did adjust the hubs before building up the wheels by using a setup similar to Sheldon Brown's to simulate QR skewer tension with the hubs in truing stand.

Unfortunately I cannot remember with certainty that the play wasn't there when I first installed the wheels, though I would assume that I did feel the rim for bearing play at the time. And had there been play at that time I would have adjusted it out per my usual procedure. Hence I have the overwhelming impression that something in fact changed between the time when the wheel was first put into use and the later time when I noticed the play.

As for QR adjustment, when I noticed the looseness in the bearing I did try to take it up by tightening the QR lever, but the lever was as tight as I always make it, and there simply was no way that I could tighten it more, not enough to remove the play, anyhow.

And thanks for the thought about spoke tension affecting the hub shell, in any case. That had not crossed my mind.

I believe that I will take the time to inspect the races before re-adjusting the second offending front wheel. Wear does seem like a likely enough explanation -- if I throw out everything I thought I knew about cup and cone hub ball bearing durability, that is.