I know now what the problem is; you are talking about SMS in "conversations". I don't use my N900 as a phone much, so "conversations" means instant messaging to me.

When I browse through the conversations, all I see is conversation I had on various XMPP accounts, not one single SMS-thread. I wasn't even aware until I checked thread by thread.

I now started an SMS thread from my phone to my N900 and the result is exactly as you describe. It says "You" in this case.