When I plug an USB harddisk, it is correctly detected:

usb 4-1: new high speed USB device using ehci_hcd and address 3
usb 4-1: new device found, idVendor=059b, idProduct=007e
usb 4-1: new device strings: Mfr=56, Product=63, SerialNumber=88
usb 4-1: Product: Iomega HDD USB 2.0 Drive
usb 4-1: Manufacturer: Iomega
usb 4-1: SerialNumber: 03000B5AE5F4
usb 4-1: configuration #1 chosen from 1 choice
scsi1 : SCSI emulation for USB Mass Storage devices
usb-storage: device found at 3
usb-storage: waiting for device to settle before scanning
usb 4-1: reset high speed USB device using ehci_hcd and address 3
scsi 1:0:0:0: Direct-Access     ST325082 3A                    PQ: 0 ANSI: 0
scsi 1:0:0:0: Attached scsi generic sg0 type 0
usb-storage: device scan complete

The problem is that udev do not load sd_mod anymore, and
"the disk" properly said, isn't detected. Loading it by hand works:

sd 1:0:0:0: [sda] 488397168 512-byte hardware sectors (250059 MB)
sd 1:0:0:0: [sda] Write Protect is off
sd 1:0:0:0: [sda] Mode Sense: 33 00 00 00
sd 1:0:0:0: [sda] Assuming drive cache: write through
sd 1:0:0:0: [sda] 488397168 512-byte hardware sectors (250059 MB)
sd 1:0:0:0: [sda] Write Protect is off
sd 1:0:0:0: [sda] Mode Sense: 33 00 00 00
sd 1:0:0:0: [sda] Assuming drive cache: write through
 sda: sda1
sd 1:0:0:0: [sda] Attached SCSI disk

Perhaps it's related to the following udev change:

* Mon Jun 04 2007 - kay.sievers@suse.de
(...)
- drop scsi module loading rules, the kernel has modalias strings now

I'm running with the latest factory updates, on PPC. Unfortunatelly I
can't test it on my x86 machine because it has SATA disks, and sd_mod
is already loaded by default.