By default in 10.4, Spotlight won't index the content of mounted disk images. If you're like me and keep your mail (and other sensitive documents) on an encrypted disk image, none of that content is searchable from Spotlight -- whether from the Finder or even in an application like Mail.app -- because all searching is done through Spotlight indexes.
To enable indexing on a read/write disk image, you can use 'mdutil' (meta data utility) from the Terminal. Here's the command:
sudo mdutil -i on /Volumes/name_of_image
After a few moments (depending on how much data is in the image), you'll find your mail in Spotlight results. You can also do a man mdutil to see the rest of its commands.

