VMWare fusion allows you to share folders from the host operating system with the guest operating system. Under Ubuntu 8.04 and 8.10, the permissions are not set correctly to give read/write access to the share.
The share itself is located under /mnt/hgfs in Ubuntu. For some reason, the owner keeps getting set to 506 and the group to dialup. To fix this, edit /etc/fstab and look for the VMware entry (it will be commented). Look for the part of that line that starts with the word default, and add this right after it (after the word default, not that entire section):uid=1000,gid=1000. Save the file, and reboot (or unmount and remount that share).
This assumes you are the only user on the virtual machine, in which case your user id and group id are 1000.
I use this to share a Pictures in my home directory on OSX with Ubuntu for using Digikam.