I think scrolling using the middle button + TrackPoint was broken for a while, although it is now fixed. to fix it instead of modifying your xorg.conf, create a new file called /etc/hal/fdi/policy/mouse-wheel.fdi with the following contents:
<match key="info.product" string="TPPS/2 IBM TrackPoint">
<merge key="input.x11_options.EmulateWheel" type="string">true</merge>
<merge key="input.x11_options.EmulateWheelButton" type="string">2</merge>
<merge key="input.x11_options.XAxisMapping" type="string">6 7</merge>
<merge key="input.x11_options.YAxisMapping" type="string">4 5</merge>
<merge key="input.x11_options.ZAxisMapping" type="string">4 5</merge>
<merge key="input.x11_options.Emulate3Buttons" type="string">true</merge>
</match>
Then restart your computer.
I have tested this method on my X200 and it looks working fine.
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