Would it make sense to also allow items to be tagged by dragging a tag onto one or more items? I'm rather used to the Gmail label model, and find myself trying to do this quite often (to no avail).
Well, dragging a label to a selection of emails works in Gmail, but there label-to-email is the only way to go, and the concept of (email) selections is somewhat stronger because of the use of checkboxes to mark selected emails.
I think dragging multiple objects onto something is much more of a standard convention than dragging something onto a multi-selection.