If you'd like to use SAI, then the programmer HardCyder has made the tutorial for you:
In this tutorial he describes the process of installing all the required tools to get SAI working with pen sensitivity, which I know lots of artists have been asking for. If you know other artists who would use Ubuntu if they could get SAI working, then this might be a good opportunity to encourage them to switch over.
As a Free Software advocate, I will of course encourage you to use Free Software alternatives like MyPaint or Krita in your Ubuntu installations. But of course I understand if you need to use SAI instead and this might help you.
What do you think?
Happy Painting, Martin