So I just checked, it seems I've not gone mad.
The installer behaves differently for amd64 and i386.
For i386, it skips the "edit MBR/use entire disk" step and goes straight
to the disklabel step after I've chosen the sets to install.

OK, probably this will fix it (I'm not sure whether the names ar OK,
I just copied from i386):

--- distrib/utils/sysinst/arch/amd64/md.h.orig	2006-02-26 11:25:52.000000000 +0100
+++ distrib/utils/sysinst/arch/amd64/md.h	2007-04-02 20:43:16.000000000 +0200
@@ -73,6 +73,14 @@
 #define SET_KERNEL_GENERIC	SET_KERNEL_1
 
 /*
+ * Disk names accepted as valid targets for a from-scratch installation.
+ *
+ * On amd64, we allow "wd"  ST-506/IDE disks,  "sd" scsi disks, "ld" logical
+ * disks, "ed" IBM ESDI disks, "raid" raidframe disks
+ */
+#define DISK_NAMES "wd", "sd", "ld", "ed", "raid:no_mbr", "xbd:no_mbr"
+
+/*
  * Machine-specific command to write a new label to a disk.
  * For example, i386  uses "/sbin/disklabel -w -r", just like i386
  * miniroot scripts, though this may leave a bogus incore label.