I got some message over talk that I want to copy to clipboard. That is not possible. Long pressing does nothing. Tried on 4.0.4-ICS and 2.3.7-Gingerbread without succes.
If you are using Google Talk and have a GMail account, your chats are automatically logged in GMail. If you search for label:chat, you should be able to find that conversation and copy the text from your GMail application.
BTW, I think that the fact that you can't copy the text from the chat app really needs to be fixed.