Have been trying Tiny Core Linux now, and it might work.
The boot time from a slow USB flash drive (SanDisk Cruzer 8Gb) where roughly about 20 secs from I pressed the powerbutton... (BIOS used about 10 seconds).
Since one can get a tweeked XP to boot in this time... I would say that Tiny Core is actually a setback... since so much applications are developed for XP/Windows... Ie. FusionBrain is Win-only...
A fast SSD would probably help things up, and tweaking the BIOS would surely help as well.. But I wanted to make a "as standard as possible" test witch I think is more suitable for comparison for most users.
The real big problem in my opinion is that, when you start with a small linux distro, you need to add a large amount of library for the fornt-end software (gps navigation, media play, ecc...).
If you want to save up space you can try to build a small distro with gentoo and uClibc instead off glibc.
I do it several time, but unfortunately every time many package have problem to build with uClibc.
For my carpc I'm using gentoo linux with glibc. The main reason is that with gentoo you can choose which package use, and not install unwanted package. Many distros force you to install useless dependecies.
I boot the OS from 4 8GB USB dongle in RAID5, it's not fast as a SDD but is cheaper and is not affected to the chock as the normal hard drive.
The next step is to load the whole os into ram using squashfs to increase the interaction speed (usb I/O perform not so good), and use the USB key only for media.