Fabien, I don't know if you are still updating this code, but I'll share my findings here anyway. I found your plugin very useful, but there were just a few minor bugs:
Found/tested on WP 3.1 with MultiSite enabled with SubDomains option.
1. In the Options page, when I clicked on the link to "Edit Now" (to edit the inline content) I got an invalid URL (404). The plugin was trying to take me to "... /wp-admin/page.php?action=edit&post=xyz" I changed 'page' to 'post' and then it worked properly. ie "... /wp-admin/post.php?action=edit&post=xyz"
2. The Thickbox script was trying to load images (eg. the animated loading gif) from the wrong location, leading to a ton of 404 errors. It turns out this is a bug (sort of) with the built-in WP Thickbox script. See here: http://core.trac.wordpress.org/ticket/6453
To fix this according to the instructions in that thread, I located the following line in fsThickboxAnnouncement.php (around line 242) :
And I added the following lines just before that line:
3. The plugin relies on WP_PLUGIN_URL to build the URL to load files from. However, I found that this constant is incorrectly set when using a SubDomain MultiSite installation. Instead, I replaced the WP_PLUGIN_URL constant with the plugins_url() function, and now the URL is correct.