Zombie Panic! Source is about to get a pretty huge update, the previously promised Linux version is a 'top priority' and it should come (as long as there's no delay) on November 30th.
They released some new teasers of the big 3.1 update too, here's one of them:
When talking about the release, they didn't want to release across thanksgiving weekend (who would?) so they're giving out a "very, very tentative release date of Nov 30th". As long as nothing unexpected happens, we should be good to go. The Linux version "is still happening and our top priority in this upcoming build".
It's going to be a beta build, so when it's up you will need to opt-in on Steam. It's free, so if there's issues it won't be a big problem and they've asked Linux gamers to report everything they find when it's up.
See their update here.