^ There's no real upgrade path. You'd have to set up a new Debian Lenny machine, install zimbra on it then move the accounts over. I'm curious as to why you want to ditch Ubuntu 8.04. It's what I'm currently using and Zimbra will be supporting the 64 bit version into version 7.
If you've got an itch to have the latest and greatest OS I would hold off and upgrade to 10.04 LTS once it's supported by Zimbra. The great thing about Ubuntu is that they have a set release cycle that gets you a new LTS every 2 years. Debian does not do this as they release "when it's ready"; see
here. Also, Ubuntu LTS releases are supported for 5 years
(source) while the previous stable releases of Debian are only supported for 1 year after the new stable is out
(source).