The Google Base Blog announces new updates to Google Product Search. The post explains that the product_type attribute has been added and should apply the category and values from the taxonomy listed here. Further, the blog post states more about the taxonomy:
The Google product taxonomy is a tree of categories that describe product families, with verticals (Electronics, Home & Garden, etc.) at the highest level, followed by more specific product families or products within these broad categories (such as Electronics > Audio > Audio Players & Recorders > MP3 Players).
While Google finds this value optional, I'd consider it pretty helpful and I'm sure others would agree.
Forum discussion continues at WebmasterWorld.