On Debian lenny the latest stable is outdated. As far as rtorrent and libtorrent are dependant only on common other system packages you should take the ones from testing/unstable (as they are stable releases):
Current rtorrent version 0.8.5 depends on libtorrent11 while 0.7.x depends on version 10.
Install the packages.
dpkg -i libtorrent11_0.12.5-2_i386.deb dpkg -i rtorrent_0.8.5-2_i386.deb
Setup the sample config.
$ cp /usr/share/doc/rtorrent/examples/rtorrent.rc ~/.rtorrent.rc
read here for more info: http://kmandla.wordpress.com/2007/05/02/howto-use-rtorrent-like-a-pro/