Hook for Permalinks Update

Hi all,
I’m developing a photogallery plugin for WP and I would make it compatible both with 1.5.X and 2.0.X version.

The problem I have is this:
1) if the user install the plugin while the main blog has permalinks not active, the plugin works. In the same way: 2) if the user install the plugin while the main blog has permalinks active, the plugin works (the plugin setup check if permalinks are active and if they are, add its rewrite rules to .htaccess)

The problem occurs with case 1): I’ve a blog with no permalinks, I active the plugin, and later I active the permalinks on my blog: the plugin does not work.

I would like to know if there is a hook available when Permalink Options are updated, compatible also with 1.5.X versions.

Thanks!