The lightest, smallest computer than does Lightroom *really well*. It would be 11" Macbook Air with the i5 and 4gb RAM, hands down. Fantastic machine, really quick, super light. Load it up with Lightroom 4, CS6 and Parallels with Windows 7 instead of Bootcamp.
Parallels has a feature called "coherence" mode with allows you to mingle Windows and OSX apps without switching OS's or rebooting. It works fantastic. I use MS Office under Windows 7 and I can double click on a .docx file and OSX will automatically launch Word under Windows without leaving OSX. I don't know if I'm explaining it very well, but it works really awesome. For file comparisons I still believe the Windows version of Beyond Compare works better than anything OSX has, so I just use it through Parallels. In Coherence mode, Windows can "see and write to" the OSX folders and OSX can "see and write to" Windows directories.
The Macbook Air is just an awesome device. There's a reason every Windows laptop manufacturer is trying to copy the concept -- it really is a game changer. Barely larger than an iPad and more powerful than any laptop more than 1 year old. The SSD in the Macbook Air makes everything launch almost instantly.
Do this -- go to the Apple store with a flash drive full of photos and pop it into a Macbook Air. It will shock you how fast it will load your photos in iPhoto.
take care,
JP