Is PageRank Affected by Google Links?

In the latest Google Webmaster Central YouTube Channel upload from Matt Cutts, he talks about whether or not Google places value on its own links with regards to Pagerank. Cutts took on the following user question:

Question: Does Google value its own links for PR/Linkjuice? Google Bookmarks, Google Profiles, etc. Reason - Google links never appear in Webmaster Tools.

Cutts says he doesn't know about whether Google links never appear in Webmaster Tools, but in general, people hold Google to a very high standard. He gives the example that if Google delivers search results that aren't blocked by robots.txt, people find out about it and draw a lot of attention to it, whether that be by blogging or forums, or whatever.

"We try to be relatively careful," says Cutts.

With search results, Google tries to make sure they're properties aren't too spammable, and if they are worried that something might be potentially abused, they will often use a nofollow attribute.

Cutts says with Google Knol, most of the external links started out as nofollow, but then over time as they learned which Knol authors were more trustworthy, they lifted the nofollow on those Knol authors.

"Sometimes we do flow pagerank, but we try to be relatively careful because we're mindful of the people who do try to abuse it," he says.

Google has a lot of properties, and this might not answer everybody's questions on the subject, but I would bet that the trust factor mentioned with regards to Google Knol, could probably be applied to other products.