Found out why Arch Linux was misbehaving....

....and it was totally my fault. Yes, I made a silly, inexcusable and very embarrassing mistake in my /boot/grub/menu.lst (the one in Ubuntu as Ubuntu's Grub is in the mbr and this is what I use to boot everything).
I installed Arch to /dev/sdb1 where I previously had Zenwalk. In Ubuntu's /boot/grub/menu.lst,I had placed a stanza for Arch directly above where the (now unused) stanza for booting Zenwalk was. However, instead of commenting out everything in this stanza, I only commented the "title" line which meant that no indication of Zenwalk would show up in the boot menu.
However, this meant that there was now no "title" line to tell Arch where to stop reading before it booted. So, Arch continued to read to the kernel line for Zenwalk. And what kernel would that be? Yes of course, it was 2.6.28.7 which still existed in /boot/zenwalk.
Avoiding this problem is as simple as just totally deleting the Zenwalk stanza or just commenting out every line in it.
So, what I've learned is that in multiboot systems, it's essential to ensure there is an uncommented "title" line below each OS's stanza or nothing at all.
After I rectified this, Arch began to behave normally. Indeed, given the booting mess that my error had created, it's quite remarkable that I was able to get anything at all working.