When email scripting was extended to use custom objects, I was thrilled and delighted--but unfortunately, the only custom objects that are supported are those linked to Leads and Contacts. Given that Account-linked objects are used by Salesforce.com deployments with Person Accounts, this cuts off some advanced work that could otherwise be done. Even if the object also has a child relationship to a Contact, this only works for pulling the corresponding Contact-level record rather than inheriting the Account-level record.
I realize this is definitely an edge use case, but being able to script from Account-level objects would be most appreciated, especially given the ability to pull Smart Lists based on those same Account-level objects.