PageRank is a calculation based on how many incoming links point to a file on your site. Robots.txt controls access to the files on your site. The presence of a robots.txt file on your domain will not raise or lower your pagerank since it does not impact the links pointing to you.
Your robots.tx may impact the pagerank of sites you link to if your robots.txt prevents Google from following your links to other websites. Also if you buy links from someone and that seller uses robots.txt to block Google from following the links to your site, your site's pagerank will not receive the benefit of those paid links.
For detailed discussions about PageRank you should visit the Google Forum [webmasterworld.com]