Depends what you are trying to do.

If you just want to send people the whole post in a new post notification then that is very easy, all you need to do is edit one of the templates to send the post as well. If however you want to make it an option all you will need is an extra template and an if/elseif when a new email notification is made. Doing those two mods is realtivly easy me thinks.

When it gets harder is if you want to send all posts independant of whether they have come back or not.

I am slowly working on something like this but more importantly (and harder) i want to be able to get people to reply via email so people can use vbulletin like they used to use egroups. I am really waiting for the final to get back to it though i have started. Don't rely on it though.