You know what...
If the keyboard of my W520 existed in standalone form, with the same feeling of rigidity and substance that it has as part of the laptop (read: it has to be heavy, inflexible, and stay put on typical work surfaces), including the complete UltraNav (Trackpoint and textured Synaptics trackpad), that would be a pretty decent board and I would like to have one. It would also need to be supported by the true Ultranav/Synaptics driver package, which I imagine shouldn't be too difficult, just needs the pointing devices to present themselves to the OS as being Ultranav rather than individual mice.
That being said, you'll have a hard time finding agreement between people who have many ideas of what would define the perfect version of this product. Many will say the trackpad absolutely must not be present or it's junk - I say it absolutely must be present because I alternate between them without any thought about it regularly. Then there's the question of WHICH Lenovo or IBM version of the ThinkPad keyboard you'd use - some will want the backlit current Lenovo variant which does get positive reviews. Others will want the late Lenovo version of the classic IBM design. More still will want pure IBM stuff before Lenovo was in the picture.
I like the idea, and I'd be interested in it as I have described it, but I do think it would be challenging to get consensus on the right setup. If the kit proposed was just a case and controller that kind of kills the possibility of a trackpad...depends how the controller is done and all that. It would allow for buying whichever version of the keyboard was desired, except certainly the connector has changed along the way too...