I cannot reproduce this issue. Here are the steps I took:
1. Export 'All Content' from single site
2. Created a new site on test network
3. Imported the export file letting it create new users (except for one)
This all worked as expected with my one of my users being mapped and the other four were created and had the appropriate posts/pages assigned to them.
4. Deleted all posts and pages from the import site
5. Re-imported but this time mapping to the users now existing on the site
Again this worked as expected. Let me know if you think I did anything differently. I will also try a couple of other setups when I have some more time later on.
Just to clarify the behaviour you are seeing: all users imported, all posts/pages imported *but* all posts are marked as authored by the current, importing user?
Also it might really help me to reproduce if you could send me a copy of the export file if possible. Email wp AT lionsgoroar.co.uk if possible.
[PS. minor note, the importers were pulled out of core in 3.0]