This has just started happening to me recently too. I have noticed however it only happens with longer conversations (txts over two messages etc) and where the txt history is quite long. Bat pull seems the only solution for me. Strange it only effects one conversation, and not all the messaging conversations.
I read that OS 126.96.36.1994 fixes this problem and whilst it's available on voda uk, it's not on Orange uk (.351 is the latest as far as I know).
Why have Orange always insisted on taking forever in releasing the latest updates for their handsets. They were the same with Nokia. *rolls eyes* Do they only have one software tester who works part time?
I was tempted to install the new .994 OS but I suspect that would invalidate my carrier insurance and possibly my RIM warranty if there is an issue with the handset at a later date, even if it's an official release and I don't need another 'brick' lying around!