After upgrading my X201T to Windows 8, I noticed I couldn't use my ThinkVantage button no matter what.
I found out I could install SimpleTap to enable the button, but even though I made a good use of it back on Windows 7, I don't think I wanted it on W8, especially when its sole purpose was to make the button work.
I took out my other laptops, and after some comparing and testing, I found out that adding another entry other than HKEY_LOCAL_MACHINE\SOFTWARE\IBM\TPHOTKEY\8001 does the trick.
In addition to the "default" and "file" string you have in HKEY_LOCAL_MACHINE\SOFTWARE\IBM\TPHOTKEY\8001, you need to add the same key plus a "Parameter" string(which is to be left blank) to HKEY_LOCAL_MACHINE\SOFTWARE\LENOVO\TPHOTKEY\Class\01\18
This key entry may not exist as default, so you may need to add the "18"entry to do this.
I made a preconfigured .reg file and files to make it easier to customize. If you feel a bit lazy or think my work is sleek enough to use, I suggest you just use it http://www.mediafire.com/download.php?t45x8s68lw2f274
I didn't test it on Windows 7, but since this method has been discovered by examining the registry keys of Windows 7, I think it will work on the OS as well.
Not sure also, but you may need UltraNav Driver installed. Didn't test it without the driver, though.