As I remember it, GTD does suggest having multiple to do lists based on where you would do each type of task. I think having your emails to do list in your email makes sense. If responding to the email requires you to do a lot away from the computer, than it makes sense to put it on your other to do list.

Also, I'm not sure I'd like to have email subjects, etc... on my to do list. I don't usually keep my list that granular since a lot of the tasks I need to do are pretty amorphous until I actually start on them and figure things out. Maybe its not the best way to do it though....