I may have another clue...

I noticed that I only seemed to feel the problem when I was using my
laptop directly; as it happens, I generally do that when using either
802.11 or EVDO.  By contrast, when I've logged into it remotely, I'm
almost always on a wired net.

I tried running for a while with no network connectivity, i.e., with no
EVDO card inserted and with ath0 ifconfig'ed down.  The problem still
occurred.  On a whim, I disabled wm0 -- and the problem stopped....

I'm at a loss to explain this.  wm0 hasn't been touched since November,
but the problem started much later than that.  I do know that when the
port isn't connected to anything, I'll occasionally see messages like

	wm0: device timeout (txfree 4095 txsfree 63 txnext 56)

in my console window.  However, both dmesg and /var/log/syslog show
only one during the relevant time period.