In order for version 1.12.0 (51742) of the Second Life client to run on my Ubuntu Dapper AMD64 machine, I had to do the following:
1. Download these 32-bit packages:
http://archive.ubuntu.com/ubuntu/pool/main/a/alsa-lib/libasound2_1.0.10-2ubuntu4_i386.deb
http://archive.ubuntu.com/ubuntu/pool/main/d/db4.2/libdb4.2_4.2.52-24build1_i386.deb
http://archive.ubuntu.com/ubuntu/pool/main/libo/libogg/libogg0_1.1.3-0ubuntu3_i386.deb
http://archive.ubuntu.com/ubuntu/pool/main/libs/libsdl1.2/libsdl1.2debian-alsa_1.2.9-0.0ubuntu2_i386.deb
http://archive.ubuntu.com/ubuntu/pool/main/e/e2fsprogs/libuuid1_1.38-2ubuntu2_i386.deb
http://archive.ubuntu.com/ubuntu/pool/main/libv/libvorbis/libvorbis0a_1.1.2-0ubuntu2_i386.deb
http://archive.ubuntu.com/ubuntu/pool/main/libv/libvorbis/libvorbisenc2_1.1.2-0ubuntu2_i386.deb
http://archive.ubuntu.com/ubuntu/pool/main/libv/libvorbis/libvorbisfile3_1.1.2-0ubuntu2_i386.deb
2. Put them all in a temporary directory.
3. Open a terminal window and run the following commands.
Items in square brackets should be changed to reflect your local environment.
cd [temp dir]
dpkg-deb -x [first package] .
... etc ...
dpkg-deb -x [last package] .
cp lib/* [SL dir]/lib
cp usr/lib/* [SL dir]/lib