According to Intel, the ICH8 chipset does not 'use' AHCI - only the ICH8R. However, linux (I'm using Ubuntu 6.10) does appear to. On my Asus P5B motherboard, I can set the SATA mode to AHCI, and linux quite happily recognises the disk and runs fine. This is with the disk connected to the ICH8 SATA port, not the JMicron one.
Could someone explain what exactly is going on here? Does the ICH8 support AHCI, or is linux quietly overriding the BIOS and running in IDE mode?