During a small side project I've found out that Google Rich Snippets Testing Tool doesn't treat RDFa as RDF (i.e., a graph) but rather as a simple hierarchical structure (i.e., a tree). It doesn't take under account links in RDFa, but only the way HTML elements are nested inside one another. More about the difference between data models of graph and tree can be found in a blog post by Lin Clark.
I've created two documents that give the same RDF when you run RDFa distiller on them. Both contain GoodRelations product data, but the difference between them is that in the first document the HTML element describing price specification (gr:UnitPriceSpecification) is a not nested inside the HTML element descibing the offering (gr:Offering) and the offering is linked to via gr:hasPriceSpecification property. In the second document the HTML element with price specification is nested in the element about the offering.
Even though the documents contain same data, Google Rich Snippets Testing Tool parses them differently and refuses to show a preview of search result in the case of the first document, whereas the second document produces a preview. In the first case, the price information is not recognized because it's not nested inside the HTML element describing the offering and thus a warning is shown:
Warning: In order to generate a preview, either price or review or availability needs to be present.
This leads me to believe that Google Rich Snippets Testing Tool doesn't parse RDFa as RDF, but as a tree (much like a DOM tree), effectively the same way as HTML5 microdata, which is built on the tree model. Google doesn't use RDFa as RDF, but as microdata.
Eric Hellman wrote a blog post about spoonfeeding data to Google. Even though Google still accepts some RDF (e.g., GoodRelations) after the announcement of microdata-based Schema.org, it wants to be spoonfed RDF graphs packaged as microdata trees. Does it mean that if Google is your primary target consumer for your data, you shouldn't bother with packaging your RDF in trees, but rather directly provide your data as a tree in HTML5 microdata?