linux is most definately a desktop platform... but its not really a desktop platform for designers due to the lack of good quality applications available.
ive been using ubuntu on and off for about 12 months now (going back into windows for illustrator, photoshop etc). I installed ubuntu 6.10 a few weeks ago and im extrmemly impressed, (ive not booted back in to windows yet) runs quicker than xp, boots quicker and fully customizable to whatever you want to do.
a few design apps:
inkscape
http://www.inkscape.org/
xara xtreme
http://www.xaraxtreme.org/
the gimp
http://www.gimpshop.net/
scribus
http://www.scribus.net/
ive not used any of the above yet for a job as i am more efficient in photoshop and illustrator, ill be giving them a proper run out soon though.