Note, however, as gyrog points out later on that thread, " So there is a possibility that it is working because the eSlacko kernel has improved mmc support, after all we only sorted out proper mmc support in the kernel, a few months ago." So, if you want to boot from an SDCard it's likely you'll need to use a relative recent (note the date of this post) Puppy or swap a recent Kernel into your Puppy.
See also Watchdog's post here, viewtopic.php?p=8072#p8072
[Sorry, don't have time right now to locate the thread discussing the development of mmc support].
I've kept on eye out for posts about using SDCards since my cat knocked my laptop off a table. Newton's Third Law of Motion. Whenever I boot it from a USB-Key disturbing images flash by of half a USB-Key stuck in a USB-port.
PXE boot. Here's a old post from 2007