After upgrading sarge to etch - spamassassin was installed - but sa-exim wasn't running (the headers in mail showed it to be to do with the setting of SAEximRunCond stating that it should not run).
After a lot of looking at the default line in /etc/exim4/sa-exim.conf I found lower down the line:
SAEximRunCond: 0
Comment this out if you want sa-exim to run spamassassin on the mail!
Have been running sendmail for a long time on woody. However - with the upgrade to sarge I've been taking a look at exim's configuration - since the sendmail config is a nightmare :)
Status so far
Installation
apt-get install exim4-daemon-heavy
This got exim4 down and removed sendmail. No debconf prompts came up at all. Some searching in debian-user gave me
dpkg-reconfigure exim4-config
Good start - got it listening on more than 127.0.0.1 and got the list of local names installed.
Spamassassin
I was using a sendmail milter to run spamassassin - for exim an apt-cache search gave me a hint