I'm specifically excited about productivity applications and even third party mail clients (Sparrow, etc) taking advantage of GMail emails being associated with a task.
I agree that this would be great, but I don't see anything in the docs which suggests this task-to-email linkage is available via the API. If I'm overlooking something, please let me know, as I would definitely like to use that.
Now I would really like a proper Contacts API. For custom fields, Gmail uses one format and Google Apps another. That's a big oversight, and one no one has seemed to recognize yet.