Note: This post is old. Refer to this post for updated instructions (2017).
The version of smplayer in the original Debian repos is too old. This is how you can compile the latest smplayer from sources:

Download the latest source package here

Extract it:

tar -jxvvf smplayer-

Install necessary packages as root.

apt-get install libqt4-dev zlib1g-dev fakeroot build-essential devscripts debhelper g++

Now compile into a debian .deb package:

cd smplayer-
dpkg -i ../smplayer_0.8.2.1_i386.deb