their response was:
So, apparently perl is "custom software", because the problem is CLEARLY with perl. Perl segfaults on spaceymail-mx1, but not on spaceymail-mx2 or gatorade. Just because the perl script in question is spamassassin and not some 5 line perl script that I could probably concoct so that perl crashes in the same way if I really wanted to piddle my time away, does not mean I am using "custom software". Note that my complaint is not "help me setup spamassassin" or "spamassassin gives error X", it is that perl, which is listed as a feature of their hosting, is seg faulting. That's ridiculous.
So... I finished installing my own perl (5.8.8 with all the defaults except the paths obviously), and what do you know spamassassin no longer crashes on spaceymail-mx1! I told them this, but apparently they are really busy so who knows when/if they'll fix it. I wonder whether they even lifted a finger so far on this issue, because it worked on the first try for me when I installed perl.
If you want to get it working yourself, first complain to DH again for good measure, then download perl 5.8.8, install it with all the defaults but tell it different paths as described at http://forum.dreamhosters.com/3rdparty/28969-thread-enabled-perl-breaks-Interchange.htm and then set your PERL5LIB in your ~/.bashrc to /home//software/lib/site_perl/5.8.8/:/home//software/lib/5.8.8/ and re-install spamassassin & its pre-reqs as described at http://wiki.dreamhost.com/index.php/SpamAssassin using CPAN & your new perl install.