Thank you for pointing to the Leopard "Unknown" group issue as the cause for this "Cannot map RAM: File already exists" error. This is important information.
Interesting that Apple's advice in fact recreates the Tiger group permissions with a group "username" for each user. Tiger was in that respect an exception and Leopard returns to users being in group "staff".
(Edit: The MacOSX versions that used the single user groups were Panther and Tiger. Before that users were in group staff and in Leopard again users are in group staff.)
Being aware of this permissions issue with migration from Tiger to Leopard, the first thing I did after migrating from Tiger was to make sure I (the user) was in group staff and my Home folder and its contents belonged to group "staff". (See commands below.)
Probably because I did this, I never had the "Cannot map RAM" error on my iMac.
Several forum members reported solution of this error problem after applying a 10.5.x Combo update. Could such a Combo update also correct this permissions issue?
What I did after migrating from Tiger to Leopard:
dscl . read /Groups/staff
showed I was not in group staff.
sudo dscl . append /Groups/staff GroupMembership $USER
added me to group staff.
sudo chgrp -R staff $HOME
changed group ownership on all files in home to staff.
sudo diskutil repairPermissions /
repaired disk permissions.
Finally I rebooted.
Source was this hint and discussion on MacOSXhints:
http://www.macosxhints.com/article.php? ... 1291134001