Moshu,
You can switch from the default (aka ugly) any time. BTW, the default/ugly permalinks ALWAYS work, no matter what kind of “nice” structure you set up.
Does this mean that if I started with www.yourdomain.com/wordpress/?p=100
and changed the permalinks to reflect www.yourdomain.com/wordpress/wordpress/2008/01/26/sample-post/
that both links would still work?