Ok, so this is the complete way to reflash the stock PR 1.3 kernel back into your N900:

1. Download the PR 1.3 image corresponding to your location from http://tablets-dev.nokia.com (in my case, the "Global" image)
2. If you use openSUSE on your desktop, there are two gotchas: a) "modprobe -r cdc_phonet" will unload a kernel module and let flasher-3.5 access the USB port; b) "zypper in libusb-0_1-4-32bit" if your computer is x86_64
3. ./flasher-3.5 -F RX-51_2009SE_20.2010.36-2_...something....bin --flash-only=kernel -f -R
This will reflash only the kernel