EDIT: I've sorted this now (or rather, I'm in the process of doing so) by mounting the SATA drive in my Xubuntu desktop, backing up and formatting the rootfs partition and reinstalling ALARM along the lines of
http://archlinuxarm.org/support/reinstallation. I've got it booted, updated, and I'm currently waiting for the pacman keyring master key to generate (nope - it's just finished!). Now I can set about reinstalling the packages I need and setting them up the way I had them before, which will be made easier by having the backup to refer to.
Argh! I've cocked up and found myself with an unbootable GFH. I was tired and not paying enough attention

Long story short, I did the pacman -Syu --ignore filesystem,bash and then lost my SSH connection due to stupidly closing my laptop lid. I then found I couldn't log in as anyone, including root - all 'access denied' - so, perhaps even more stupidly, I switched the GFH off and on
Since then, I've dug out my old desktop and installed Xubuntu on it, rather than p*ssing about with Windows 7 on my wife's laptop. Now I can see the uBoot messages:
$this->bbcode_second_pass_code('', 'U-Boot 2010.09 (Feb 16 2011 - 18:44:23)
UBIT v0.6 by Jeff Doozan and Peter Carmichael
(Re)start USB...
USB: Register 10011 NbrPorts 1
USB EHCI 1.00
scanning bus for devices... 1 USB Device(s) found
scanning bus for storage devices... 0 Storage Device(s) found
Hit any key to stop autoboot: 0
UBIFS error (pid 0): ubifs_get_sb: cannot open "ubi:fast", error -19
Error reading superblock on volume 'ubi:fast'!
UBIFS not mounted, use ubifs mount to mount volume first!
** Block device usb 0 not supported
** Block device usb 0 not supported
** Invalid boot device **
** Invalid boot device **
** Block device usb 1 not supported
** Block device usb 1 not supported
** Invalid boot device **
** Invalid boot device **
** Block device usb 2 not supported
** Block device usb 2 not supported
** Invalid boot device **
** Invalid boot device **
** Block device usb 3 not supported
** Block device usb 3 not supported
** Invalid boot device **
** Invalid boot device **
Reset IDE: Bus 0: OK Bus 1: not available
Device 0: Model: ST2000DL001-9VT156 Firm: CC97 Ser#: 5YD2EQ76
Type: Hard Disk
Supports 48-bit addressing
Capacity: 1907729.0 MB = 1863.0 GB (-387938128 x 512)
Loading file "/boot/uImage" from ide device 0:1 (hda1)
2888208 bytes read
Loading file "/uInitrd" from ide device 0:1 (hda1)
** File not found /uInitrd
Loading file "/boot/uInitrd" from ide device 0:1 (hda1)
2643572 bytes read
## Booting kernel from Legacy Image at 00680000 ...
Image Name: Linux-3.1.10-20-ARCH
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 2888144 Bytes = 2.8 MiB
Load Address: 00008000
Entry Point: 00008000
Verifying Checksum ... OK
## Loading init Ramdisk from Legacy Image at 01100000 ...
Image Name: UBIT-2010.11-00125-ga21bcca
Image Type: ARM Linux RAMDisk Image (gzip compressed)
Data Size: 2643508 Bytes = 2.5 MiB
Load Address: 00000000
Entry Point: 00000000
Verifying Checksum ... OK
Loading Kernel Image ... OK
OK
Starting kernel ...
')
I also had nc running for kernel messages, but nothing ever appeared on there. The power LED is solid green, but I can't seem to do anything with it.
I guess one option is to reinstall ALARM from scratch (though I need to make damn sure my files on the SATA drive don't get lost - I run my business with them!), but I wonder if the system might be salvageable? I am perhaps a bit out of my depth, as I don't use Linux that much, but if I were to mount the GFH's SATA drive in my Xubuntu system, could I make changes via the filesystem (either config files or unpacking something somewhere) that would make it bootable again?
Would appreciate anyone's advice as to the best course of action!