The mode-mouse option allows scrolling with the mouse. However, for any setting other than "off", it also causes tmux to hijack the mouse. That is, clicking and dragging does not select text (unless I also hold down shift). This is a big disadvantage for me.
I would like a mode in which the mouse behaves as close as possible to how it behaves without tmux, including some form of scrollback. I don't even care whether it's really scrolling back though tmux's buffer, or my terminal's. I just want to scroll to see my recent output in the common case. It's ok if there are some limitations. (I have been assuming all along that the program running under tmux is not mouse-aware. I don't care about mouse-aware programs.)
There is a hack for GNU screen: http://liquidat.wordpress.com/2008/10/17/short-tip-gnu-screen-with-proper-scroll-session-management-support/. I have no idea how it works, but it seems to do what I want.
I found a similar hack for tmux: http://superuser.com/a/300716/134608. But it doesn't seem to work (maybe it was only for older tmux?). At least, I see no effect. So I am posting this as a feature request.
I have tested the current tmux subversion, as well as tmux 1.6. My terminal is xterm. I am running Debian unstable.