I have tried both my own builds of Chromium OS and Hexxeh's daily Vanilla builds, and neither work. I copy them over to my flash drive, but when I try to boot from it, nothing happens except my backlight flashing on and off a few times, then a reboot. System: Compaq Presario CQ50-140US http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01550071&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&site=null&lang=en&product=3795225&key=null
There is a guide at hexxeh.net.
1.1 For Windows Use WinRAR or 7zip to extract IMG file from the downloaded archive.
1.2 Use Windows Image Writer, then select the IMG file and select the USB device from the menu.
1.3 Click on "Write".
1.4 Boot from the newly created Chromium OS USB stick!
2.1 On Mac OS - simply download the easy to use builder application for mac to make your vanilla USB drive.
3.1 On Linux - extract the IMG file from the downloaded archive
3.2 At the shell, run the following (where sdX is your USB stick and ChromeOS.img is the path to the IMG file you extracted): dd if=ChromeOS.img of=/dev/sdX bs=4M
3.3 Boot from the newly created Chromium OS USB stick!
In addition to the above, and for completeness of this answer (but which you probably know), you might have to change your boot order in the BIOS. To do this restart the computer and press F2 (sometimes F8) and change the system boot order so that your USB port is higher than the internal disc drive.
When I did this my self, my USB pen drive (Kingston Travler 16GB) got corrupted. When formatting it It would only take 1GB, I saw many partitions on it using Windows Disk management which I could not delete and neither could I extend the 1GB partition. A Google search shows that this is a common issue with the Chrome OS.