If you mean indenting according to thread parent/child relationships, no. We've found that the majority of users find that really confusing. It also has the negative effect that new mail arrives buried within the thread tree, not at the top where users can see it.

Threaded view might resurface some day for advanced users, but right now it's not on the roadmap.

We do support a (non-hierarchical) conversation, though (of course), as well as a sender-based view that's pretty handy in certain circumstances.