As far as I know, current smartphones and "smart" tablets (e.g.
android/ipad/meego/playbook) all use NAND flash memory for storing the
OS. I believe the reason your iphone boots so quickly is because you
are just waking it from a sleep state. You either have to let the
battery die completely or else hold down the power and "home" buttons
to actually get it to reboot. On the iphone 3g that I have, boot time
is around 30-40 seconds if I actually reboot it. If you are looking
for instant-on linux, you can check out the webtop OS Asus offers on
its current laptops (also any of the other various "webtop" instant
OSes that come with different laptops- their all unix-based).

For installing something, have a look at mandrivas instant on os,
though it is not free:
http://www.mandriva.com/en/instanton/

There is also a thread on the net somewhere for building a five second
bootable xandros or Arch Linux. Also, I have been told that Ubuntu
10.04 and up is supposed to boot in 10-15 seconds with sdd drives,
though on my old disk with ext3 formating, it's around 50 seconds.