libssl0.9 has moved onto version libssl1 though Spotify currently has prerequisite dependency of no more than libssl0.9 and won’t install.
You can safely install the two side by side. Download the required version from squeeze repositories and install with one of the following scripts:
sudo wget http://ftp.de.debian.org/debian/pool/main/o/openssl/libssl0.9.8_0.9.8o-4squeeze13_i386.deb && sudo dpkg -i libssl0.9.8_0.9.8o-4squeeze13_i386.deb
sudo wget http://ftp.de.debian.org/debian/pool/main/o/openssl/libssl0.9.8_0.9.8o-4squeeze13_amd64.deb && sudo dpkg -i libssl0.9.8_0.9.8o-4squeeze13_amd64.deb