Patrick Baudisch showed in Paris at the Microsoft Software summit interesting photos of their DIY activity in creating a glass floor for tracking and activity recognition. With a fairly large glass pane in the floor they have created an interesting environment… I am sure there will be interesting things coming out of this installation.
Some years back in 2002 (and looking at the photos and the amount of hair I still had this seems long ago) in Lancaster we also looked in what to do with floors (and we were into DIY as well). We also considered arrangements with a floor tables and furniture on top. As you can see from Kristof, Hans and me on the photo it was a fun project.
The positive point in using load sensing is that you can track unobtrusive and potentially large scales with little instrumentation. We even considered the possibility to put a house on 4 load cells and do activity recognition based on this. We never got around to building the house The problem with load sensing is that you can only track one moving object/subject at the time.
Looking at the signature of the load measured and doing some signal processing we could detect events – unobtrusive and cheap – but only for single events.
Interested in more details? Have a look at the publications on load sensing , on the interaction , and at a patent  describing the basic technology.
 Schmidt, A., Strohbach, M., Laerhoven, K. v., Friday, A., and Gellersen, H. 2002. Context Acquisition Based on Load Sensing. In Proceedings of the 4th international Conference on Ubiquitous Computing (Göteborg, Sweden, September 29 – October 01, 2002). Springer-LNCS, London, 333-350.
 Schmidt, A.; Strohbach, M.; van Laerhoven, K. & Hans-W., G. 2003. Ubiquitous Interaction - Using Surfaces in Everyday Environments as Pointing Devices, Universal Access Theoretical Perspectives, Practice, and Experience (UI4ALL 2003), Springer LNCS, 263-279.
 Schmidt, A., Strohbach, M., Van Laerhoven, K., Friday, A., Gellersen, H-W., Kubach, U.; Context acquisition based on load sensing. US Patent 7434459. US Patent Issued on October 14, SAP AG (DE), 2008