[exim] Question on DKIM

Tuesday I upgraded a server from 4.68 to 4.72rc2. Last night I got
the following in my paniclog on that server:

2010-06-03 02:52:36 1OK5EC-0003AM-0q DKIM: Error while running this
message through validation, disabling signature verification.

This caught me off guard because I didn't intend to enable DKIM and I
don't reference it anywhere in my config. I don't have an
acl_smtp_dkim defined. Is this the intended behavior? It doesn't
seem to follow the principle of least astonishment.

I see now by reading 54 that I could disable DKIM by setting
DISABLE_DKIM in Local/Makefile, but since that's not in src/EDITME, I
had no intention of turning DKIM on, and that's not mentioned in
NewStuff or ChangeLog, I never saw this. I think that
DISABLE_DKIM=yes should be added (commented out) with an explanation
to src/EDITME. I will add this unless someone objects.

That said, even knowing that, I probably wouldn't have turned it off,
I probably would have included it to play with later, I'm just
surprised that the inclusion of the code without referencing it
anywhere in my config has side effects. Is that just me not
understanding the nature of DKIM?

Thanks
--John