I have two experiences with Atom based PCs and Second Life. They may be considered good or bad according to the level of expectations
I have run SL on an Atom based netbook with Intel integrated graphics at the lower settings with a "light-enough" viewer (the old snowglobe). At low settings it does work and it is possible to move the avatar smoothly. Of course the visual quality is sub-optimal. I used SL with Win XP and never with Ubuntu, but the experience with other applications is that the netbook feels much more responsive away from Windows.
I have also run SL on a nettop with an Atom/ION chipset. I used both Vista and Win7. The SL experience may be a pain at medium settings , but it is possible to use it (I even went sailing). Visually is much more appealing provided one gives enough time to the textures to upload. Again, I am sure Ubuntu would allow more processing power to be available to the Application.
A caveat is that in both cases I have used V1 viewers. Not sure how much more a V2/3 viewer would take.