I recently got an Lenovo T400 and this thing works great with the latest version of Ubuntu (Intrepid).
To install Ubuntu, you have to make sure you edit the BIOS settings and change the graphics to use one of the 2 build in graphics cards. If you want to save the Vista bootloader and access to Lenovo’s restore partition, you must not install grub on the MBR (Master Boot Record). Instead, install grub on the same partition as where the kernel resides. (This will be either /boot or / ) Then install EasyBCD¹ on Vista and add Ubuntu to Vista’s bootloader.
I was going to install Debian on the laptop, but I went w/ Ubuntu Intrepid instead as it has a 2.6.27 kernel which (supposedly) has better support for some the hardware on the T400 (and for my new Sony MP3 Walkman). I could have installed Debian and compiled my own 2.6.27 kernel, but these days I couldn’t be bothered spending ages compiling, testing, and debugging my own kernel.
¹For instructions on how to use EasyBCD see: http://neosmart.net/wiki/display/EBCD/Ubuntu