Problem updating an AD user

[egholm]

I'm trying to update a user in my AD with the following code:

DirectoryEntry de = GetUserFromAD(user.Domain, user.LoginUserName);

if(ImpersonateValidUser("User", "Domain", "Password"))
{
de.Properties["givenName"].Value = "testtest";
de.CommitChanges();
}

I'm impersonating an user that has write-access to the AD.
My code dosen't throw any exception, but the changes never take effect.

Help