booting OpenDarwin 7.2.1 (and Darwin 7.0.1) via GRUB

I realize this is a well worn path, but I have been unable
to persuade installs of OpenDarwin 7.2.1 and Darwin 7.0.1
on an x86 (Athlon XP) PC to boot.

The setup is:

disk0: WinXP, Fedora Core 2, Debian, swap
disk1: Gentoo, OpenDarwin, Darwin, swap

and I am using the Fedora GRUB to boot.

Both OpenDarwin and Darwin installed happily from CD
onto UFS partitions. The GRUB stanzas used to attempt to
boot are:

title OpenDarwin (7.2.1)
        map (hd1) (hd0)
        rootnoverify (hd1,1)
        chainloader +1
        makeactive
        boot

title Darwin (7.0.1)
        map (hd1) (hd0)
        rootnoverify (hd1,2)
        chainloader +1
        makeactive
        boot

I also tried inserting rd=disk1s2 and  rd=disk1s3 into the appropriate
place in /Library/Preferences/SystemConfiguration/com.apple.Boot.plist
but both OpenDarwin and Darwin are getting no further than

        map (hd1) (hd0)
        rootnoverify (hd1,*)
        chainloader +1

when attempting to boot.

Oddly, a similar setup on another PC (i..e Darwin on the second
disk) _is_ booting quite happily.

Any inspiration would be welcome :)

Des Johnston