Guys, pagerank minus one is returned for websites which are yet to be indexed by Google, or purposefully omitted by Google from its indexing system. Reasons may vary: such as found violating the Google TOS and subsequently expelled, or the website home page itself having some sort of restrictions e.g. <meta name="ROBOTS" content="NOINDEX,NOFOLLOW"> Simple as that!!!
Page rank algorithm which allot a value to every web page which is vary from 0 to 10 and new web page has unranked value but minus 1 never. Check again that. It is impossible to get minus 1 page rank still. So don't get confused with that PR meter and it is fact that PR meter is not perfect PR meter.