Alan,
I dont think there will be a 2.8, at least not branched from master.
Whatever is in master will be 3.0, there's just too many changes in master to backport to stable branches.
When 3.0 will be released is up for debate; all development is driven by the users of Cobbler....
At the very least all refactoring and generalizing work has to be done before we can even think of stabilizing and cutting a release.
There will be some more 2.6 releases though, but no fundamental changes will occur in that branch, just minor fixes and feature improvements.