I just spent 2 hours reviving my Aspire One netbook after inadvertently killing it whilst fiddling about configuring dropbox. I found the whole process unnecessarily fiddly and information on the interwebs to be a bit scarcer than I would have liked, so I'm documenting it here in case I need it in the future. Hopefully it'll be useful to someone else too.
So, the cause of death was a typo when trying to set up the dropboxd daemon to start automatically on boot. I'm not running nautilus so couldn't use one of the prepackaged releases, and it's completely my fault that I made a mess of installing the vanilla x86 build.
After making the fatal change and rebooting, the system would only boot up to a blank black screen with a default X mouse cursor. This is because the system was trying to run my broken command, failing, and therefore never getting to the main desktop.
In the world of normal linux, there's all sorts of ways of dealing with this, but despite plenty of googling I couldn't find a way to use run-level 2 or 3 on an Aspire One, and the Ctrl+Alt+F1-F6 key combos for switching away from X to a terminal don't work either. There seems to be no way of preventing the system following the same doomed process over and again if you break X.
Frustrated, I thought about using the restore disk, but that's a nuclear option - it re-paves the whole machine, so bye-bye data. That seemed a bit drastic when all I needed to do was edit a single text file to fix the system.
Ironically, this was happening as a result of me trying to install a file sync system as a simple backup. Grr.
Still, like countless thousands before me, I was saved by a live linux distro - in this case, a USB bootable one (since the Aspire One has no optical drive). Following the instructions1 at pendrivelinux I created a bootable Feather Linux USB drive, and booted the netbook from it by hitting F12 on the post screen and selecting to boot from the USB stick.
At the boot prompt, I used 'knoppix 3' to boot the system up to a command line, mounted /dev/hdc1 as an ext2 filesystem, and fixed my typo. Reboot, and tada! Everything was working again (well, after hitting Fn-F7 to reenable the touchpad, which I had accidentally disabled whilst mashing the keyboard in frustration at the sight of a blank screen about an hour earlier, heh).