Phoronix: A KDE4 Operating System In Less Than 200MB
SLAX 7 is now up to a release candidate state and it packs a KDE 4 desktop environment while the entire operating system is less than 200MB in size...
KDE is 25MB
KDE dependencies 60MB
KDE applications and such are 17MB
The rest is core Linux system tools, 50MB
ARM version was planed for a while, but I discovered that it's not such universal as like x86. There are many ARM devices out there and each one (as far as I understand it) needs different firmware to boot. I can't possibly support all ARM devices, so I would have to pick just few, and that makes no sense to me.
Else it is actually pretty simple to make Slax for ARM, all sources are available on FTP and Slackware (which is base for Slax) already has official ARM version as well. Unfortunately I don't have any fast ARM device here, just some RaspberryPI which has just 256MB, it's not possible to recompile KDE4 on that... it needs around 4GB of RAM for compilation (yet it runs just fine on 256MB of RAM when compiled).
A quick google search actually turned up what looks to be a very relevant result (a how-to/script for apt-based distros which will set up scratchbox2, qemu, and a rootfs for raspberry pi cross-compiling):