Well, having used a few different Technologic products, they can boot in around 2 seconds. In practice though, once you incorporate services such as USB, boot time increases quite a bit. Their 7350 board will boot to a shell with USB in around 6 seconds. If you want full Debian, that takes well over 30 seconds. The 4900 board which uses Yocto will come up with USB and Wifi connected to a network in about 10 seconds. They're using a Freescale processor on that one. Pretty nice product.
Of course, another factor is what file system you're using. If you're using something like Ext2 and you don't gracefully shut down, you might be forced into an fsck on the next reboot which can take who knows how long. If you don't have any sort of status display for the boot sequence, that's a problem.