You can do it with a combo of apt and source compilation. The sources aren't in the apt repositories, but you can download them from the webpage, and then compile them the same as you would the debian source packages.
First off, it is easiest to let apt install all the dependencies -
Code:
apt-get build-dep rtorrent
The get both the libtorrent and rtorrent packages from this page -
http://libtorrent.rakshasa.no/
Untar them as root in /usr/local, doing libtorrent first, then rtorrent should be fine. Since you compiled them outside of apt, apt won't keep them up to date, but you'll be able to just compile the new version of the source.
If you have trouble compiling, write back and we'll be able to help with that as well.
Peace,
JimBass