If you look at this topic in the forum: http://forums.gentoo.org/viewtopic.php? t=4634 There are some information on how to install gentoo on hardware RAID controllers. If somebody could test it with both Promise and HighPoint controllers, so it can be tested if it works or not. And added to the installation instructions.
Link to topic not working, just put the 2 pieces together to complete the link.
Bug 7543 has a nice page of info *** This bug has been marked as a duplicate of 7543 ***