I am not surprised. I am about to post a New Topic about this, here is the information:
Despite an Apple article to the contrary at:
http://docs.info.apple.com/article.html?artnum=300896
Spotlight will not file text within the content of files on disk images created in Disk Utility, which are already mounted on the desktop, whether or not they are encrypted.
Forcing indexing using this command: sudo mdimport [filepath]
does not help either.
Nor does using command-F to get the detailed version of Spotflight and using the + button to select "content".
Spotlight only searched the names of files and folders on these disk images, not the content.
If you have a different experience or a workaround, I'd greatly appreciate hearing from you. I use encrypted disk images extensively and not having this functionality really hurts. I keep my mail on this type of image and now can no longer search by content.