It seems that Merb's Mailer class is either using a local sendmail client or a non-TLS enabled SMTP server. This is not a unique problem to merb, but rather a deficiency in Ruby 1.8.
I took some time to look around and found that Rails has the same problem, and it was fixed via a plugin, not a gem as is the "merb way". There was also a gem that packaged the Net::SMTP classes from Ruby 1.9, which do have TLS support. It isn't hard to guess what I did next.
I monkey punched Merb::Mailer to overwrite the net_smtp method and added two config options into merb_init.rb. See the pastie for the code example here.