How can I convert first page of a pdf as its thumbnail?
Some people have started a Drupal 7 version, but it's nowhere near ready yet. If you can program for Drupal 7 and want to get involved, you could check out their Git repository and help them out.

If you're not ready to start porting D6 modules to D7, you could try one of these options to build those thumbnails, then write a little snippet of code to attach them to your nodes in your imagefields:


Stack Overflow: How do I convert a PDF document to a preview image in PHP?
Webvamp: How-to create PDF preview images in PHP