<s:states>
<s:State name="button" />
<s:State name="map" />
</s:states>
<s:Button includeIn="button" click="{currentState = 'map';}" />
includeIn="map"
Yann,
You are running into the same issue that the Login widget had. Look at the code changes that I made to the viewer to handle this. Ultimately it is a timing issue.
http://forums.arcgis.com/threads/25899-Login-Screen-for-the-Flexviewer?p=139178&viewfull=1#post13917...