Whilst on holiday I found an N900 in the sea.
My heart skipped a beat until I checked my pocket and found out that it was not mine. Quite a co-incidence!
Anyway, I rinsed it of salt water and dried it thoroughly.
I was very pleased to find that it does boot, I can hear the "NOKIA" jingle, and when I touch the screen I feel the feedback-vibration, so the O/S is running.

The screen, however, is black.
The composite video output does not work, either.

I've disassembled the unit and cleaned the ribbon connectors to the screen with solvent, to no avail. Upon reassembly it was still black.

I have been able to succesfully connect it to a PC and flash it with the factory ROM, so I've no doubt that the operating system is OK in there.

Is there any way I can get some use out of this unit?
If I could get sshd or VNC installed somehow, I could control it as a headless device and perhaps use it as a Wifi audio device or something... but I'm at a loss as to how to install sshd/VNC blind.

You can (theoretically) create a new rootfs image using backupmenu and mtd-utils (see http://talk.maemo.org/showthread.php?t=63975). The eMMC image should also be doable (see http://talk.maemo.org/showthread.php?t=74121).

You could back up your existing N900 with backup menu, and try to blindly restore from that? There's a rescue flash image somewhere related to the backup utility that has the base + backup app installed. With that, you should be able to clone your existing device, which would give you ssh and more.

Edit: Crap.. someone beat me to it.

... and we can assume you took it by the local nick to make sure the original owner wasn't looking for it?

Holly JESUS!!!

I droped mine in a toilet and flushed it... and here it came out in some neutral waters out...

I recently had a similar situation where my screen's ribbon was severed 2/3 through, to the point where I could input touch screen input but no backlight, screen output, indicator light or ear speaker (the small speaker you listen to while on the phone). I eventually purchased a broken N900 off ebay with a broken screen which I swapped into the original, but I was able to issue commands through therminal by hitting SHIFT+CTRL+X for xterm. you may need to run a few commands just to be able to ssh since you restored. I've been able to blindly connect to my wifi and ssh, vnc and the rest from here.
piping output to espeak helped some

Opening the terminal with SHIFT+CTRL+X is a great idea, although I would need to input my WiFi credentials before I could connect to it, which might be tricky.

The cloning of my working N900, replete with credentials, seems like an easier option.

And in reply to 'demolition': given the number of pirated videos I found on the MMC card before I flashed it, I think I did the owner a favour by not taking it to the nick!

Did you try to connect it to an external screen/tv? You then could see if touch does any thing at all.

Yes, I did try connecting a composite video lead and that did not work.
I think the touch functionality does work, though, because I feel the vibrating touch feedback when I press the screen.

Ok sir, You got VERY lucky..
OT: Why not just get a new screen or clean the insides?