amd64 cannot boot on ASUS M2N-E

[Angelo Turetta]

The motherboard (BIOS rev.0203) is based on the nVidia MCP55 chipset, 
AMD64 3500+ (2200 MHz), 1 GB DDR553, two SATA2 Maxtor 80GB, DVD-R Samsung.

Trying to boot the latest snapshot 6.1-STABLE-200608-amd64, the kernel 
probes most peripherals (the Marvel gigabit-ethernet is not detected, 
but that's a known item), but just after reporting acd0 it hangs solid 
with the IDE led on. No panic, nor errors.
It seems related to the detection of the disks (they are not reported). 
I even left it in that state the whole night (just in case it was some 
loooong timeout), but nothing.
The -i386 image exhibits the same exact behaviour. Booting without ACPI 
does not solve.

7.0-CURRENT-200608-amd64 boots fine (no Marvel gbit, of course).

What should I try next?

Angelo.