There is an option in the 'schapman advanced configuration tool' for this, search for this in google, comes in first in the results list. I have not tested it though, not got enough text messages yet.
I had the 'schapman advanced configuration tool' for my TyTnII and it was a useful bit of code!
However I have a problem on my HD with the threaded messaging, in that it only showed the last message that I had received from a contact.
I thought this was a threaded messaging thing so turned it off with the schapman tool and now it appears to "overwrite" the previous message so that I only ever have the last message received from that contact in my SMS inbox. I have looked for the others with no joy.
Any help would really be appriciated.