Problemer med mod_rewrite og $_GET variabler

HTML kode 

 <form method="post" action="?action=login" class="loginform">
 <p>                        
 <label>Brugernavn</label>
 <input name="username" class="textbox" type="text" />
 <label>Kodeord</label>
 <input name="password" class="textbox" type="password" />
 </p>
 <div style="color: red;"><?=$login_errormsg;?></div>
 <div align="right"><input class="button" value="Login" type="submit" /></div>        
 </form>