OK people, here are my experiences that you might find useful
Obviously, first of all make sure you've got the second preview release correctly written to an SD card and inserted into the board.
I contacted support, you need to hold down the power button for ten seconds for it to boot. For whatever reason, if you just press it, or hold it for any shorter length of time, it just turns off.
The running status of the LEDS is bright green at the front with a dim red next to it and at the back of the board next to the HDMI socket, a red LED.
There's a group of components on the back of the board that are positioned in such a way that while holding the board to plug in the power, or to pick it up perhaps - it shorts some contacts - this leaves the board in a kind of 'crashed' state - if you unplug the power, wait a moment, and plug it back in, being careful not to touch the board - it'll be in a normal bootable state.
hold down the power button for ten seconds etc
the screen goes white - then will either go to a linux post, or you'll get no HDMI signal - if you press the reset buton, the RGB lights on the front light up just like when you press the power button, the board resets and it consistently works correctly if it didn't before.
I've been very methodical, so if your experiences differ let me know so we can establish the normal operation of this board so any problems become apparent.
For example, after booting, there doesn't appear to be a registered audio device for the HDMI audio output for me - perhaps this is a pending driver that wasn't mentioned that's currently not working.
Either that or there's a potentially problem with my board or cables.
Also, whenever I shutdown the board, the lights just stay on - pressing the power button after it's on, or holding it down for any length of time - does nothing - you have to unplug the power and plug it back in to get back to the 'lights off' state. The reset works fine - so perhaps the power button requires something additional to function correctly - but there doesn't currently appear to be any non-software way to return the board into the 'lights off' state for me.
oh and the board doesn't remain on at all without an SD card inserted - no matter how long you hold the power button, the lights go out. I've not yet setup the board to use eMMC - the instructions say to remove the SD card - perhaps it'll remain on without an SD card in that configuration.