The short simple answer.
Rewriterule ^(.*)$ http://www.somedomain.com/$1 [r=301,nc]
This rule will match anything after the /
and the 2nd part of the rule will take that match and add it after the domain name of www.somedomain.com
and then redirect that new URL. The new URL will be shown in the address bar.
For what you want to do, that rule won't work. You will need to do this.
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
Rewriterule ^(.+)$ index.php?page=user&p=$1 [L]
This will be an internal redirect so that what shows in your address bar will be a URL like
http://somedomain.com/somename