hmm.. I am starting to think that maybe they are not deleted until you empty your trash. decided to test it and via USb cable attached to computer, deleted some files. unhooked the Droid and looked at the SD card and had - IIRC - the same "used" file size of about 3.33 gb I had before I deleted the files. Hooked it back up via USB and deleted trash on my computer, unhooked Droid and checked again and now at 2.61 gb.
So, I am thinking if you delete using an app on your phone like Astro, deletes are instantaneous BUT if your deleting via a computer and have the Droid attached via USb cable, it will not delete until you empty your computers trash. Or so it appeared to me now that I tested it.