A dot in the URL is seen as the extension separator, which the Framework strips off. The only way to work around it is to disable 'strip_extensions' in the routing section of your config.php.
The Observer_Slug uses Inflector::friendly_title() to generate the slug.
And as far as I can see that strips the dot from the string passed, both in ASCII as in UTF-8 mode. So how your slug can contain a dot, I don't know. Which version of Fuel are you on?