Yes, I think it was Steve Jobs comparing iPad to desktops saying "desktops are like trucks", meaning you use them for the heavy lifting but not for anything fun. But this may change when productivity software catches the Touch paradigm. Remember when desktops were just for hobbyist use and the real work happened on terminal applications accessing mainframes?

You can also have MeeGo or Android running on the N900. And besides that, it is sweet to have a phone where you have root.

I'd say, the concept of desktop as it was defined through 80s and 90s is beginning to die. Touch interfaces, actually well-working mobile devices and web services ("the cloud") are taking over more and more of the desktop's traditional role. More than a problem for the Linux desktop, I see this shift as a big opportunity as the importance of the traditional vendors like Microsoft is declining. Here are some ideas on what the "Linux desktop" ought to do: http://bergie.iki.fi/blog/the_web_and_the_free_desktop/