Fatal error: Call to undefined function format_string()

format_string is new in Drupal 7.9, so you should install Drupal 7.9.

Aside: I did not find a way to specify a dependency on a minimal minor version of core in the .info file. I will document it though on the project page.

Use hook_requirements() to check whether the Drupal installation is version 7.9 or higher.

I think I had a look at it back then and found that it is not possible to define a minimum minor version for core modules. But please correct me if I'm wrong.

hook_requirements() can do anything you want. The core property in info files can't.

I see, you're right of course. but IMO this is to much work for almost nothing: who is still below Drupal 7.9 but does upgrade this module?

If you can write a patch I will add it, but I'm afraid I don't have the time to do it myself.