Congrats with the plug, I got mine last monday
My situation is a bit different from yours, but I'd be interested in the answer also.
I'm running Debian on an SD card (followed http://www.cyrius.com/debian/kirkwood/sheevaplug/
), and the kernel and initrd are both on the SD card and booted using an upgraded uboot. First thing I did after getting the plug was running the alpha 6 installer, and after that the Debian install guide. Now I can easily switch kernels by using the standard debian proces. No SD performance problems here. (I do have big problems when I attach a usb hub between the plug port and external hard drive, but I'm working on that
Anyway, I'm also wondering how to install the "cbxbiker61" kernels. For example, it delivers a uImage, but in my Debian install a kernel is always accompanied by its own initrd (or, after uboot gzip, uInitrd), no idea yet how to generate that, unless there's an easy way to 'gunzip' the uImage.