on a single boot machine...
I know it involved installing refit on its own partition.
anyway, did a search and could not find.
*edit:* Found it. This thread may be deleted.
where are those instructions for getting Ubuntu to boot faster
Archived thread 1016536 from Apple Hardware Users. Markdown source: Apple_Hardware_Users/thread_1016536_where_are_those_instructions_for_getting_Ubuntu_to.md
You don't necessarily need rEFIt; see http://ubuntuforums.org/showpost.php?p=5166788&postcount=21 in the FAQ.
It worked great for me.