last answer to this was 2+ years ago... just curious if there is an update?
I think you might be able to use siftrock.com for this with its Marketo integration.
There is definitely still not a trigger that acts on email replies - but Dave is right. Siftrock is a tool that can solve some (but probably not all) of this need that probably wasn't around 2 years ago.
Siftrock solves this situation entirely. It's great. use it.