Here is an introduction on why you would want a separate /boot partition.
The advice I have been given is that 200MB is a good size that will accommodate several installs of Ubuntu(or other GNU/Linux).
The only caveat that I see to this method of handling multiple installs of linux is that you can't just overwrite the main / partition to remove or upgrade an existing install of Ubuntu. You will now have to ensure that old kernels are removed from your /boot.
This page offers a more thorough explanation, which I will look into, and hopefully update this page with my findings.