As a potentially really dumb “you-should-have-known-better” experiment, I put Karmic on my work-issued T400. Also probably inspired slightly by Alex Payne’s post of his relative success with Ubuntu on his X301.
Well, on the plus side, it mostly works. Screen comes up in the right resolution. Volume/brightness keys work.
But it’s still not quite complete. There are still problems that you can work around relatively easily, and ones that you plain can’t.

• When I plug in a monitor, my multi-mon layout makes a deadzone (normal), but stupidly I can move the pointer into that deadzone, making it really hard to click on any panel element that lies along that deadzone (fitt’s law.. grr). So basically, if you want multimon in linux, make sure you don’t have deadzones (i.e. have monitors of the same size)
• Trackpad sensitivity is too high. But if I tweak the settings in gnome, then external mouse sensitivity suffers. There’s actually a separate touchpad tab in the mouse settings, but no slider for sensitivity there.
• Battery life still seems to suck compared to windows.
• UI for configuring external monitor sucks. Also seems like monitor plug/unplug detection doesn’t work. It only notices if I load up the layout config applet.
• No HDAPS.. I guess you can kinda blame Lenovo for continuing to do something non-standard here, but alas, it’s still a deficiency.
• Undock is weird, according to thinkwiki.
• Suspend and resume doesn’t work. Sigh.

The HDAPS stuff can apparently be worked around. And I think the trackpad issues you could deal with too. So the bottom line is, if you don’t ever plug anything into your laptop then you should be good to go.
I suppose this is an improvement compared to my first experiences on the X60. But there are still fundamental usability issues, and fundamental “just work” issues. Then again, if actually I cared about that, I shouldn’t be running Linux, should I?