After quite a bit of trouble I've managed to boot linux from SATA RAID on my Asus P4P800DX, which has onboard software RAID support using the Intel ICH5R southbridge. I've written down the steps I've taken along the way and have put them online for all to try.
The URL is http://users.pandora.be/TheBlackUnicorn/linux/
I hope someone can benefit from this. The process is not that straightforward but it works nicely now. All comments are very welcome.