I had a similar problem, I've not fully diagnosed it, but I was seeing this in the /var/log/messages file:
Apr 20 12:54:33 ip30 kernel: usb 2-1: new full speed USB device using address 2
Apr 20 12:54:34 ip30 kernel: usb 2-1: device not accepting address 2, error -71
This was on a self-built kernel (2.6.11) for SuSE9.2. If I booted the original SuSE kernel, it worked, and usb0 became available.
After a bit of googling, I wondered if it were the particular USB port on my motherboard, and switched it to a USB1 port from the USB2 (where it was plugged in more for convenience/accessibility than any other reason). And guess what!
Apr 20 12:57:43 ip30 kernel: usb 1-1: new full speed USB device using address 4
Apr 20 12:57:48 ip30 kernel: usb 1-1: control timeout on ep0in
Apr 20 12:57:48 ip30 kernel: usb 1-1: Product: Zaurus SL-C7x0/C860
Apr 20 12:57:48 ip30 kernel: usb 1-1: Manufacturer: Sharp
Apr 20 12:57:54 ip30 kernel: usb0: register usbnet at usb-0000:00:1d.0-1, Sharp Zaurus, PXA-2xx based
Paul