Ok, I went and looked on the page, and my u-boot was rather old - I updated it and now Arch boots. However, my Maemo menu entry no longer works - I need to figure out what changed in the boot menu files.
ITEM_NAME="Maemo 5 with 2.6.28-omap1" ITEM_KERNEL="2.6.28-omap1" ITEM_OMAPATAG="1"
ITEM_NAME="Maemo 5 with kernel-power 2.6.28.10-power51 (Internal Nand)" ITEM_KERNEL="2.6.28.10-power51" ITEM_OMAPATAG="1" ITEM_MODULES="softcursor bitblit font fbcon"