Here's a website in which Secure Empty Trash is discussed along with some other methods of totally deleting files.
http://www.techtalkpoint.com/articles/how-to-secure-erase-files-from-your-hard-d rive-in-osx/
Note the third paragraph under "Shredding" which says:
"As of 10.3, Apple made it easy to securely erase your data, and in 10.5 they made it even more easy. Basically, once you have moved whichever items you need to securely remove to your trash, via drag-and-drop or using the Finder “Move to Trash” keyboard shortcut: Apple Key + Delete, then select “Secure Empty Trash” from the File menu in the Finder.
In 10.5, this can be done the same way, or by using the respective button in the windw tool bar having opened the Finder window for the trash by double-clicking the Trash icon in the Dock.
And so, once that button has been clicked, OSX will run a 35-pass overwrite of the file, essentially going well beyond the basic recommendations of any government security department. And once it is done, there will be no going back, even the most persistent Mac Digital Forensics expert or Data Rescue Professional will NOT be able to get that data back from that hard drive or memory store for you."
I don't know how reliable this information actually is but I know from my own experience that Secure Empty trash takes quite a long time to delete a single page document, plenty of time for a 35 pass overwrite, it seems to me.
Rick