you sure you did a full export then? at the top of an export file you should have stuff like this
yes, if full=y is the necessary argument then i did a full export.

I am thinking, that this is a specific 8.1.7 behaviour! Could anybody confirm that?

Because if i do a full export on version 9i (with the exact same statement as i used for the 8.1.7) i can find the "create user" statements on the .dmp file (like you postet, davey).