Oh. I've got the same problem.
Nokia N810, with lastest Diablo + updates and other packages.
Last night I was in airport waiting for my parent's flight.
I was browsing Wi-Fi, watchin TV (thru Wi-Fi), listening to music and browsing the flight information (heh, I was too lazy to go to check airport monitors, so I was sitting and browsing airport's website by my Nokia N810, that's why i love it )
The flight has delay so I had to entertain for one more hour. My battery shows still (less than 1 hour). No problem - when they landed I clicked the awfully small power button and select 'Power off!' and then I puted it in case in my pocket.
After we go home I just plugged the charger and I quickly notice that I didn't see "charging icon/text" but I thought that it's not problem, because the battery need some charge to startup LCD or something (you know...like some mobile phones).
This morning I was terrified when I didn't see any signs of loading of my device...no LED no LCD - nothing.

I've tried with no success:
1. Removing the battery for 5-10 minutes
2. Pushing on/off button for more than 40-50 seconds.
3. Quickly pushing on/off button - 20-30 times....many times.
4. Gently pushing on/off button.
5. Everything from 1-4 with and without charger plugged.

There is absolutely no signs of charging/booting etc...
No corner color LED, No LCD display, no sounds...
Then I started to read all the topics in here about 'no boot', "don't off", "off now can't on" and there was hole book about this issues...but I didn't any solution.
I tried with plugging the USB to my PC to test if it is completely bricked or something.... I plugged in and it shows two new devices...on...and off....and again...this pop-ups every 10-15 seconds. But the LCD display and LED didn't show any sign of activity.
Then I tried if the Nokia Flash wizard recognize it. The wizard correctly recognized it and says 'Nokia n810'.
Then it says that I have the latest version...but if I want...it can re-flash it again. So I decided to try it...risky...(I don't know how much battery i have) but...I tried... The wizard downloads the latest binary and flashed it successfully. Then I tried to on/off but - still there no sign of life... I started to think, that my display is broken somehow, but I didn't get the "ding" sound on boot too. Next I tried with 'freezing' solution....I tried with 3 minutes - didn't work, I tried with 5 minutes - didn't work too.
The only sign of life that I notice it is when I slide out the hardware keyboard sometimes (in the on/off button "game")
I can see that the keyboard light is fading in/out about every 10-15 seconds...which I think is the same as the USB period...for 'connected' 'disconnected' message when USB is plugged in PC.
Then I suspected my charger, then I tried with my phone charger - it's Nokia too and has the same specifications - Output DC 5V/890mA. No signs of charging...or booting...etc..
I tried again with flashing. It was flashed again successfully with latest Diablo...but no difference at all. It looks like a bricked...;-(

Today I left my battery out - this is my last hope. If it didn't help I'm really running out of clues what happened. The problem is that unit is from UK, and I got it from eBay...in excellent condition. So I cannot go to service. It worked perfect for week until today. I don't know what can be the reason. Is it possible for slide on/off keyboard to cut some wires, LCD's for example. Yesterday I had some 2-3 times that the hw keyboard didn't opened so easily, but then it worked again...and I don't know....anyone have some idea what happenned?