I just finished a fairly painless upgrade from Zimbra 5.0.18 GA FOSS on Debian 4 (Etch) to Zimbra 6.0.4 GA FOSS on Debian 5 (Lenny). All in all the process took about 3 hours, including a pretty lengthy fsck. Keeping all the notes and steps for posterity's sake.
Step 0: Backup!
Step 1: Install prerequisites
Step 2: Download everything
Step 3: Run the upgrade from Zimbra 5.0.18 to Zimbra 6.0.4 Debian 4
Step 4: Upgrade from Debian 4 (Etch) to Debian 5 (Lenny)
Step 5: Run the Zimbra 6.0.4 Debian 5 installer
Step 6: Apply any optional configuration changes
