Before the N900 was released I am pretty certain I heard/read that conversations with one contact would appear in the same thread. However, this is not the case on my device. SMS and instant messages from different accounts are each kept in a separate thread. I was under impression I would see SMS, skype IM, yahoo IM, etc from the same contact in the same thread.

Is there an option to have them merged (can't find that) or was the marketing speak false or did I misunderstand the marketing speak?

I don't think you can merge all the conversations together in one box. I have several conversation box (with the same contact) with sms and skype and i cant merge the two together. Maybe you had misunderstood the marketing speak?

I believe the im contact has to be in the same field as the phone number for it to all merge together in conversations. in otherwords, they cant be 2 separate contacts. add a field for im and have all of them together.

I was under the same impression when i watched demo videos of the n900 before it was released. When i recieved my n900 i was a little disappointed to find that this was not the case. Its not a huge issue but it is a little annoying when i start a conversation with a friend over SMS and they switch to gtalk. I know all I have to do is open a new chat window but still, i would like everything all in one place so i could read the conversation in its entirety later.

I do have all my contacts set up as pegasus stated: one contact card with their gtalk info as one of the contact methods so that will not solve the issue. I think to truely have one thread conversation per contact would require changes to the maemo messaging client or the use of a third party messaging client.