Assuming you are running Magento 1.8 Enterprise, you may have noticed that the home page does not cache by default, even though System -> Configuration -> Cache Management -> Page Cache is set.
Only catalog and product pages make use of the full page cache by default.*
*Update: On a second Magento installation, with different modules installed, this was not true. On the installation where this problem occurred, it appears the core module handles the homepage request.
Run the find command to see which full pages are cached (REQEST – yes misspelled) indicates the page was cached.
find var/cache -name “*REQEST_*” -print
In the file app/code/core/Enterprise/PageCache/etc/config.xml, you will see the section which you can configure to get other pages cached. The relevant section is as follows: