Screamer (21.09.2012/21:02)yes u can use id too, but i never try it
jimy, maybe can use id of user
for example: http://site.com/id_number_of_id
or use subdomain
http://nickname.domain.com/
so more beautiful in my opinion
and for sub domain may be u can try using this code for htacces
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^([^\.]+)\.domain\.com$ [NC]
RewriteCond %1 !^(www|ftp|mail)$ [NC]
RewriteRule ^user_(.*?)$ /users/profile.php?wl=$1 [NC,L]