Is it possible to retrieve SMS messages that have been deleted from my iPhone? I have read a lot about this online and would like to confirm once and for all.

Is it possible to retrieve SMS messages that have been deleted from my iPhone?

I know it is possible to get access to the sms.db file but that only holds the current SMS messages. I tried that and was successful in getting it off the phone but when checking the contents there were no deleted messages.

And this would not be relying on any backups either, i.e. I backed up my phone, sent a message and then deleted it without backing up before deleting. Is that SMS gone for good?
You could try to recover information from the sqlite file using for example a hex editor. Here is a quote from SQLite FAQ:

I accidentally deleted some important information from my SQLite database. How can I recover it?
If you have a backup copy of your database file, recover the information from your backup.
If you do not have a backup, recovery is very difficult. You might be able to find partial string data in a binary dump of the raw database file. Recovering numeric data might also be possible given special tools, though to our knowledge no such tools exist. SQLite is sometimes compiled with the SQLITE_SECURE_DELETE option which overwrites all deleted content with zeros. If that is the case then recovery is clearly impossible. Recovery is also impossible if you have run VACUUM since the data was deleted. If SQLITE_SECURE_DELETE is not used and VACUUM has not been run, then some of the deleted content might still be in the database file, in areas marked for reuse. But, again, there exist no procedures or tools that we know of to help you recover that data.

Hope this helps.