Two years ago this month, I reported here on my experience in installing Mathematica 8 in Linux (http://ubuntuforums.org/showthread.php?t=1636057).

Now I can report on installing Mathematica 9 in Linux.

The best thing to report is that this installation went even more smoothly than my previous one. This time I simply followed the current installation instructions on the Wolfram Research Website: http://support.wolfram.com/kb/10652

Mathematica 9 for Linux downloads as a script file: Mathematica_9.0.0_LINUX.sh
I used the default locations in the script in installing it on my desktop, but it would have been easy to change to other locations if I had wanted to do so.

The installation script concludes with a hyperlink to instructions for activating the new installation: http://reference.wolfram.com/mathema...thematica.html
I had no trouble with the activation (well, that is, once I started using my Activation Key for Mathematica 9 rather than the outdated Key for my Mathematica 8 installation).