We don't disable Varnish cache since ... well without it, you're not getting any benefit of DreamPress, really.
The catch with DreamPress is that while we allow any plugin and theme out there (except hacked ones really), not all work equally well on Varnish'd servers. You don't have any domains on DreamPress right now, but if you'd like to tell me which domain was having the problem, I can look and see if there are any known conflicts.
We're working on making a list of plugins and themes we know not to play well with our setup. Part of why we're still in beta.