So, I'd already double-checked my kernel source was up-to-date (by
comparing a sup and a cvs tree) before posting.  

It turns out that doing rm vm_*.o *_pager.o made things work again.
I have no clue what's going on, other than possibly some interference
between the time I last compiled a kernel, and the modification times
produced by sup maybe causing vm_object.o  not to be recompiled.

Thanks to everyone who answered.