I got an IdeaPad S12 for Christmas! I'm going to install Ubuntu Karmic on it. This page will record my adventures.
The IdeaPad S12 has a Nano processor by VIA. It's a 64-bit, x86 compatible processor, so you can either install 32-bit x86 or 64-bit x86. Since 64-bit is still flaky I went for 32-bit.
The default installer for Ubuntu Karmic (including both the Netbook Remix and normal versions) don't contain the correct drivers for the VIA Chrome9 HC3 IGP. You'll know if this is happening to you because the kernel will boot (you'll get to stare at a pretty Ubuntu icon for several minutes), and then the screen will either go blank and have a blinking cursor, or because the screen will start flashing through colors such as red, green, blue, grey as if it were going through a factory test. I also saw a root prompt shortly before graphics attempted to kick in. You should be able to get a terminal using control-alt-fX.
There is a PPA with updated drivers that worked for me. To install it, get a terminal (using control-alt-f1) and then:
add-apt-repository ppa:xorg-edgers aptitude update aptitude full-upgrade
I did not have to custom compile the drivers myself.
Wireless did not work by default, Ubuntu offers you a choice of Broadcom restricted drivers (according to this thread the STA driver is higher performance), so make sure you have ethernet and go install it.
Works out of the box.