Well I set up gmail and sent a message, the person responded, I see that but then I (from my computer) sent another message, part of the same conversation. A day later away from my computer I could not remember if I had send my response. the n900 did not show my latest reply in the conversation so I waited went online and looked at gmail (not the n900 version) and it did in fact show my reply.

so what is up with this? does the person have to respond to my last message (or conversation as google calls it) in order for me to see the last thing I sent to them?

Or you could go to the "Sent" folder and open the conversation, there you can see what you've sent to other people.

problem is the conversation does not exist at all in my sent folder. I wrote the email from gmail on my computer, and so it does not show up in sent on the n900. I have no idea if it should. anyway so for certain I can see the last part of the conversation (sent by me) on my pc but not on the n900's gmail app. hmm.

I don't think it works that way, you'll have to use GMail in a web browser to see that(since the N900 downloads the mail and store it separately from the server).