This is what I use Jabber for--Jabber messages have an implicit VSRE attached. In practice, it's just like email except that the replies are usually short. For me, Jabber messages get delivered to the same places on both my computer and phone, and take about the same amount of effort to respond to.

It has the added benefit of potentially being a real-time conversation as well--this sometimes happens with a flurry of short emails, but it's much more awkward.

I was hoping Google Wave would take off an neatly combine the two. But it never did :(.