When you are changing web hosting providers there are two factors would determine if you lose Pagerank....

1. Are you keeping the same domain name?
2. Are you keeping the same URL and file structure?

Basically if you answered yes to the two top questions your Google Pagerank will remain intact. If you switch domain names you have to make sure you 301 redirect all the old URLs to the new ones to transfer over the Pagerank, also this will prevent broken bookmarks and broken links from other sites. If you are changing your URL or file structure you also need to make sure you 301 redirect all old URLs or filenames to the new ones.

Due to unforeseen factors and changes in the Google search algorithms you still might lose some Pagerank and never notice, this is due to the fact that Google Pagerank changes real time behind the scenes, but the public Pagerank visible in the Google Toolbar and Pagerank check websites is only updated ever 3 to 4 months.