I registered in this forum just to confirm that camiloml's BIOS in post #15 worked perfectly to enable AHCI for my P5K/EPU. This came in perfect timing, as I bought an SSD this week (Crucial M500) and wanted to take advantage of it.
Before the update, my menu only let me select IDE. Now it shows IDE and AHCI. I'm going to just dump some information in here for anyone that might want to update their P5K/EPU with this..
I used the built-in EZ Flash 2 from the BIOS tools menu, and it let me flash to it, even though my version, and the modded version were both showing as 0604.
Once you select AHCI, there's a screen at the end of POST that initializes the AHCI mode, and Windows starts (but crashes during startup unless you install the AHCI drivers first). I switched back to IDE and used this from Microsoft (fix it for me), and it worked flawlessly after switching back to AHCI:
http://support.microsoft.com/kb/922976 Error message occurs after you change the SATA mode of the boot drive (Microsoft Fix it 50470)
Windows starts up and then installs the AHCI drivers, and reinstalls the drivers for the devices that are connected as SATA (hard drives, optical drives, etc). A reboot is required after this, and then you're good to go!