Finally the 6 X-Plane 9.21 DVDs are here!! Yeah!!
I moved straight to the installation. You get the first disk to your
DVD-ROM drive and double click on the "Installer_Linux" file. No need for any command line string or anything! Just double-click and go!
A nice graphical interface starts and you go as usual: select the folder in wich you want the installation, then the scenery areas you desire (BE CAREFULL here, the full global scenery takes almost
70GB of disk space!!! If you rarely fly on a specific area, you can save space by not installing the corresponding scenery. Nevertheless you can add and remove any area you like at any time after the installation) and hit next. The proccess starts and from time to time it will ask you to insert the next dvd. At my system it took ~15minutes with ~50GB of scenery.
After the installation, launch X-Plane by navigating to the folder of the installation and double clicking on"X-Plane-i686" file. Yeap, no launcher is created! However, you can create one: go to desktop (or wherever you want to place the launcher), right click -> create launcer -> and in the "Command" field browse to "X-Plane-i686" file (eg /games/X-Plane/X-Plane-i686).
When I had tried the demo, the launching of the installation and -later- the simulator had
an issue with one of my external hard disks (!!) that prevent it from starting. So, I had to remove one of my external HDs in order to run x-plane!!
This did not happen with the installation of the full version of X-Plane 9.21! Everything worked fine and flawlessly!
However, the first launch of the simulator diged up a small issue: Upon launching, an error showed up and x-plane closed. The error was this one:

After a little search in x-plane.org folrums, the solution was found: as fool as it might sound, there isn't any host in the host list of Ubuntu to point to your own computer (127.0.0.1) !! So, you have to create a host that points the address 127.0.0.1 to your computers name (in my occasion "vasilis-desktop").
In Ubuntu 8.04 this is an easy task: left click on the network icon on your taskbar, select "manual configuration", unlock the dialog box and go to the "Hosts" tab. There add a new host with IP Address 127.0.0.1 and "Aliases" your computer's network name.
That's it, now X-Plane launches!!