Help! I don't get vmcore dumps anymore..

I haven't had a panic for quite a while until this morning, and was
surprised to see it boot up without savecore(8) grabbing the dump. 

I have a feeling that I have configured my kernel incorrectly - should I
have KDB_UNATTENDED to have dumpon work?

Here is dumpdev setting from rc.conf, swapinfo and kernel conf:

> $ grep dump /etc/rc.conf 
dumpdev="/dev/ad4s3b"
tom_at_zoot '15:52:07' '/usr/home/tom'
> $ swapinfo
Device          1K-blocks     Used    Avail Capacity
/dev/ad4s3b       2097152   186600  1910552     9%
tom_at_zoot '15:52:10' '/usr/home/tom'
> $ cat /usr/src/sys/i386/conf/ZOOT 
include         GENERIC
ident           ZOOT
machine         i386
nocpu           I486_CPU
nocpu           I586_CPU

options         SC_PIXEL_MODE
options         SC_HISTORY_SIZE=1000
options         VESA
options         SMP

device          crypto
options         GEOM_ELI

nooptions       INVARIANTS
nooptions       INVARIANT_SUPPORT
nooptions       WITNESS
nooptions       WITNESS_SKIPSCAN
options         ALT_BREAK_TO_DEBUGGER

device          atapicam

device          vt
device          drm
device          i915drm

options         NDISAPI
device 		ndis

Thanks

Tom