PageRank works almost exactly the same way as a PKI. The only difference is that instead of people signing your key to vouch that it belongs to you, they are linking to your webpage to vouch that it has quality content. PageRank can be faked, certainly, but it is difficult enough to at least significantly slow someone down. To make it more trustworthy you'd probably have to modify it to create certain webpages that were absolutely trusted, and then do some sort Kevin Bacon rank where end users were scored based on the degrees of separation. That way there is some designated starting point, rather than the whole system being based off popularity.
