How to install new tmux 1.8 on Ubuntu 12.10 or 12.04?

Here is the way I did it on Ubuntu 13.04 (Raring Ringtail) and 14.04 (Trusty Tahr):

sudo add-apt-repository ppa:pi-rho/dev (source)

sudo apt-get update (freshens the cache/database of packages)

sudo apt-cache show tmux (*optional, shows you available versions, 1.7 & 1.8 should be listed)

sudo apt-get install tmux

tmux -V

12.04 AND UP:

sudo add-apt-repository ppa:pi-rho/dev
sudo apt-get update
sudo apt-get install tmux

OLD ANSWER:

As you possibly already realized, tmux version 1.8 is too new. As of writing this answer, version 1.8 is only 19 hours old.

If you for some reason need a newest version, then you might download the code and build/install tmux by yourself. Should not be a big deal, since tmux has dependencies only on libevent and ncurses packages, and both of these packages are present on most if not all of Linux distros. Just download the latest code, and type following via terminal:

$ sudo apt-get install libevent-dev libncurses-dev pkg-config
$ autoreconf -fis
$ ./configure
$ make
$ sudo make install

I have not found an "official" PPA for new tmux packages. However a PPA search on launchpad turns up quite a few options.

Many are out of date, but I found a ppa by pi-rho that has current tmux and tmux-next packages for 12.04, 12.10 and 13.04. (At the time of this answer they are at versions 1.8-1ubuntu1~ppa2 and 1.9~20130425~bzr3062+9-1ubuntu1~ppa0). There is no guarantee that that PPA will continue to be maintained up-to-date, but for now it seems like a fair option.

Be aware that it also has a number of other packages in it. If you add this PPA you will find the other packages also get upgraded unless you do some sort of apt pin config.