Hard hang with u3g and -current

I have a Huawei E169 3G modem which I'm using with the u3g driver on
-current/i386 from about 4 weeks ago on my Aspire One (dual-core
N270).  If I access the stats port (/dev/cuaD0.2) then the system will
randomly hang.  The problem seems to occur more frequently when I'm
using X but I've also seen it (once) when I was using a VTY.  I'm not
using hald.  There doesn't seem to be a problem if I don't access
/dev/cuaD0.2.

When the system hangs, the 3G modem hangs up (though this may be the
keepalive timer expiring at the remote end) and there's no response to
the keyboard (Ctrl-Alt-Del and Ctrl-Alt-Fn from X; Ctrl-Alt-Esc and
Alt-Fn from VTY; CapsLock, NumLock in either mode).  The kernel still
responds to pings over ethernet and I can change brightness via the
keyboard so SMM is still working.

Has anyone else seen anything like this?

Can anyone suggest a way to track this down?  I can't get to DDB, there's
no serial, firewire, PCcard or similar I/O.

-- 
Peter Jeremy