I'm also getting this with spamassassin 3.1.7 which I just installed (I've been with dreamhost for 1 week). It happens with about 50% of emails for me. I don't think it is caused by emails intended to crash spamassassin as it happens with several legitimate emails I received also and if I run spamassassin manually on any of the spam it chokes on, everything works fine.
BTW, -11 is SIGSEGV, so this is a seg fault.
I tried running this using the procmail test procedure described at http://partmaps.org/era/procmail/mini-faq.html#debugging under "Help, I get this error message ... ", "Troubleshooting tips" on a message which crashed spamassassin and it did NOT crash. So something funny is going on.
Hopefully dreamhost can figure this out soon, because their "junk filter" version of spamassassin lets a LOT of spam through.