hstore is certainly an option, as are the crosstab functions in the tablefunc contrib module, which can definitely do what you are looking for. The 2-query form of the function would allow you to generate a resultset that gives true/false/null for every possible attribute without having to have a column for every attribute, yet you'd have all of the convenience of a normalized schema representing attributes when doing other queries. There are a fair number of threads about using the crosstab functions in the archives of this list.
