Its part of OS 5. You cant turn it off. You can view them all by hitting ALT-S when yoiu're in the SMS list though, but no way of totally or setting it to always be like that.
It's nice but not everyone likes it.. I do not like it as well because it's broken and if you have too many threads with 50+ messages it freezes as well. Also sometimes it'll screw up the order...
I haven't had problems with it yet. It was weird though. When I first upgraded to .484, my open conversations in BBM transfered over out of the backup as SMS. I know they look almost the same, but I wasn't mixing them. They came over as SMS. I went to carry on the conversation we had be having in BBM about 2 hours eariler and sent a message. My friend's response: why are you sending these as SMS?
It was weird. After I eneded the conversations that came with the restore I haven't had it since.