With the release of Entourage 2008 you can now have spotlight index your Entourage email attachments. But this is not on by default. To be able to search your attachments using spotlight you need to turn on this option in Entourage. You can turn it on by going to Entourage > Preferences.... On the left side pane go to the General Preferences section and then select Spotlight. You then enable the indexing of attachments checking the box next to Include attachments in Spotlight search results and then saving your changes.