I'm in the process of installing my OSes on a new PC. I've installed Ubuntu 8.04 so far, and intend to install Suse 10.2 too (I won't go into why I doing it, but I am!). Obviously, I'll have a common /home directory, but will likely use different users for each distro. Is there any other parts that can be shared such as /usr/local, or will it just be too much hassle and give conflicts?

Configure /tmp to be cleared out when you reboot, and you can share it as well.
The swap partition can be shared.
In a similar theme, you probably will want to update grub using the same distro. You can modify /etc/grub/menu.lst to boot up to either distro; or perhaps install the second distro's MBR to its own partition instead of the drive MBR and chainbooting the second distro. This will prevent a dueling distros situation where updating grub in one distro causes the other not to boot.

You don't want to share /etc/ of course. However, since the users home directory is listed in /etc/password, you can use the same username but use different directories.
For example: /home/davemar-ubuntu and /home/davemar-suse.

I personally don't think it's a good idea /home. The only thing I'd share is swap, and even then it only makes sense if you never use a "hibernate" feature (which writes the resume image to swap).

I don't put any "data" in /home, only preferences files and some scripts. The "data" and "media" files go into a big partition mounted on, say, /media/sda5. The various Linux installs are aware of this partition only insofar as it's listed in /etc/fstab. That way, no distribution specific cruft ever gets installed onto that partition, and there are no conflicts.