Erlang isn't a magic bullet. It might make migrating processes fast, but it won't speed up migrating data.
Could you explain why to me? I just can't remember. I think I might be suffering from mnesia.
Until now I only knew that erlang can span multiple hosts - but only when you wrote your application that way?
What has really helped us in terms of machine migration (but at the cost of additional overhead) is running stuff inside Xen and OpenVZ virtual machines. Those can really be migrated seamlessly - often without a noticable downtime.