Url rewriting

[eluser]Unknown[/eluser]
Hi everyone, I have some problems with code igniter url rewriting.
I want my users have personnals urls like that: http://username.mydomain.com
I have set DNS wildcard and in my vhost configuration.

My problem is for Rewrite rules.

Usually (without CI), I do something like that:
RewriteRule (.*) index.php?user=%1 [L]. So, http://test.domain.com was the same as http://domain.com?user=test

But with CI, it's not possible to do the same because i can't pass variable with index.php?variable=whatever

If I do something like RewriteRule (.*) http://mydomain.com/user/%1 [L], it works but it redirects and I want to keep username.mydomain.com in adress bar instead of mydomain.com/index.php/user/username.

Does someone have any idea to do that?
Thanks a lot :-)