Our work in this area, "Scalable Semantic Web Data Management Using Vertical Partitioning," appeared in the VLDB Conference in Vienna in September. It showed that using a column-oriented database, along with this property representation, allows us to overcome the overhead of representing NULLs, while providing two orders of magnitude better performance than the naive triples representation. This is particularly true when processing queries that must access many triples during execution (e.g., computing the number of books grouped by subject area or institution.) Of course, there is a fair amount of subtlety to getting good performance out of such a representation. Have a look at our conference paper for the details!
This paper and others was mentioned previously. This view, of property tables in column databases, is very similar to untyped relations.