Steam won't launch under Ubuntu bug
According to this bug report, it's due to an outdated library.
How to fix it
Rename or move the file libstdc++.so.6
The exact location of that file may vary depending on your system.
On my system, to find that file, I did:
sudo find ~/.steam -iname 'libstdc++.so.6'
Rename or move these two files.
Steam should launch correctly.