Text messages don't actually get deleted
You may think you've deleted them but there still there. I found out myself when I was using spotlight search to find something and noticed that some text conversations I made months ago were showing up!
These text messages were deleted a long time ago or so I thought.
When I contacted apple customer support, they told me the only way to permanent remove all text messages and ooh yes also emails , is to do a master reset, or restore to factory setting on iTunes. This intern removes all data from your phone. The downside to this is all your contacts , apps, and settings are erased. You also can't backup from previous because then it puts it back in ur phone. I know I have done it.
imac, Mac OS X (10.6.2)