same as you, I am also user of software RAID (Intel ICH7), aka 'fake RAID'. The only solution people could give me, involved the installation of SL3.4 x86_64 on a non-RAID harddrive, tempoarily, and then manage to transfer the complete installation to the existing RAID hdd .. Well, as I am not experienced enough to do so, I was forced to try out other distros and after crawling though 100s of forums I figured out that this kind of problem is very common and thousands of people have it. It seems that there are only two Linux-distros in the world support existing software-RAIDs nativly, OpenSUSE and Fedora. I picked the last one for a test and found out that worked, for me.
As far as I learned about Linux there must be some way to manage the installation of SL on a software RAID. One way would be to recompile the Linux kernel and enabling its RAID capabilities during that process. Linux kernel 2.6.22 (kernel.org) actually supports many kinds of RAID systems, even a wide spectrum of software RAIDs. The problem is that these kernel options/modules are disabled in the original SL distro. So I assume such an adapation would be sort of complex, considering that SL makes use of imagefiles on the DVD, what makes such work even more challenging. I am relatively new into Linux in general, so in my case it might take some time until I manage to get such things done by myself - or - there might be some future SL release, opened to all sorts of RAID systems, what would be really cool.