Files in Mac OS X generally have have an owner user id and group id, and permissions for their owner, members of the group, and all others (Wikipedia).
You should make sure that your user accounts on both systems use the same user id. The default group on both systems is probably
staff for most files (which is OS X internal), so that shouldn't be a problem.
You can configure the location of your home directory as well as your user account ID from the Accounts preference pane in OS X. Just right-click on a user, and select Advanced Options…:
Ensure both user accounts have the same numeric user ID (but don't change the current home directory owner's user ID, unless you want to change all these permissions), and point both user accounts to the same home directory, e.g. on a third partition used by both systems, or even on one of the system partitions, like
/Volumes/SLPartition/Users/username. Alternatively, as you said, mount a partition at
fstab -- FileVault uses a similar mechanism (mounting a disk at
/Users/username), so you should be fine doing that.
Just make sure both systems have another unrelated account with administrative privileges in case something goes wrong with your configuration. This way, you can more easily change settings around.
Remember that OS X and integrated applications (Safari, Mail, iCal, Address Book, etc.) might not be too happy sharing parts of
~/Library. The same goes with some applications, even third party: Be sure to use compatible application versions wherever possible.
Be sure to create a complete backup before doing this -- There might be rather unexpected problems. You could solve them by e.g. symlinking problematic files and folders back to each systems main disk, e.g.
/Users/Local-Username, and keep system-specific data locally.
Depending on what exactly you want to do, it might be a better idea to just symlink
~/Music, etc. to another partition.