After my talk at the Minerva School Roy Weinberg pointed me to a paper by Chris Harrison and Scott Hudson  - it also uses audio for creating an interactive surface. The novelty on the technical side is limited but nevertheless the approach is interesting and appealing because of its simplicity and its potential (e.g. just think beyond a fingernail on a table to any contact movement on surfaces - pushing toy cars, walking, pushing a shopping trolley…). Perhaps having a closer look at this approach a generic location system could be created (e.g. using special shoe soles that make a certain noise).
There is a youtube movie: http://www.youtube.com/watch?v=2E8vsQB4pug
Besides his studies Roy develops software for the Symbian platform and he sells a set of interesting applications.
 Harrison, C. and Hudson, S. E. 2008. Scratch input: creating large, inexpensive, unpowered and mobile finger input surfaces. In Proceedings of the 21st Annual ACM Symposium on User interface Software and Technology (Monterey, CA, USA, October 19 - 22, 2008). UIST '08. ACM, New York, NY, 205-208. DOI= http://doi.acm.org/10.1145/1449715.1449747