How to force default category when multicategorized posts ?

HI !

When writing a post and need to have it in several categories, there is an issue : it is NOT possible to force the default category, which will be the "canonical" one and default slug.

This is needed for many uses :

- featured categories on sliders in themes (can be temporary)
- fix the canonical slug SO the default cat.

WP seems to attribute the default slug very stangely (thought it was alphabetical but no...) so is there a way or plugin to fix that ?