ActiveRecord failure, or something else?

I'll try not to complicate this more than need be. I have been working
on a rails project for several months. Not long ago, I decided to
upgrade to rails 2.1.0 and ever since then I've been getting an error
while taking one particular action. And not just an error, I get:
Illegal instruction: 4 (core dumped)
And the whole process dies. So here's the details:
FreeBSD 6.0
ruby 1.8.6 (2007-09-24 patchlevel 111)
Rails / ActiveRecord 2.1.0
Mongrel 1.1.5

The error always occurs when I'm iterating over items that are joined to
my model using has_many :through, and I assume this is part of it. Since
this started happening I tried changing the db adapter that I was using
(from the standard PostreSQL adapter to the postgres-pr one), as well as
changing databases from Postgres to MySQL, and neither of those changes
made any difference. However, when I revert back to rails 2.0.2 (in the
environment.rb file), everything works again! This leads me to believe
that the problem is within ActiveRecord, but I can't find anything about
that anywhere.

Anyway, here's one instance of offending code. Anyone have any ideas?
Thanks

From the model:
has_many :family_members
has_many :families, :through => :family_members

From the controller:
@person = Person.find(params[:id])
@person.families.each do |f|
   result += f.identifier + "<br />"
end