I am not aware about target+ExactTarget, but there's an integration between exacttarget and AEM, where one could personalise the email body and measure with AA tags. To take email customisable a step further, Once could look at Target smartLayout in AEM, never really used it for emails though, but would be good to see how things turn out.
It would be interesting to see it in target, however I think pushing content out of Target is a rightful responsibility of AEM. I could be wrong, its just my thoughts.