WordPress Pagerank Tools plugin by mahype requests the PageRank of your wordpress urls and stores the pageranks in the database.
There is no cron option.
Function overview
Overview of all urls with pageranks
Storing PageRank history of all sites
Random requests of PageRanks
Requires WordPress Version: 2.9.x or higher, Compatible up to: 3.0.1
Download and info: http://wordpress.org/extend/plugins/pagerank-tools/