Somehow I managed to get Chrome into full-screen mode, some automatic gesture that's natural within OS X that you do without thinking… that sensibly placed button in the top-right-hand corner.
Could I get out again? Not naturally. I started poking through menus, trying to find some switch to get my desktop back to normal. Nada, though going through the settings made me think about how much Chrome logs you into Google-space.
I did manage to google my way out, thanks to OSX Daily. Apparently it's a known issue with OS X Lion and Chrome. To sum up, hitting Command+Shift+F in Google Chrome swaps the screen mode to get you out of full-screen.
I had to hit Command+Shift+F at least twice - once to bring Chrome into hell-mode where it takes even more of your eyeball space, then stepping back into good-desktop-citizen mode again.
The complete asymmetry of the swapping modes is what got me. Once in, harder to get out. I don't expect that sort of behaviour in an application, though I'm sure I've been guilty of writing code that works that way.
The fun fact is that the OSX Daily article appeared in August 2011. I don't know the Chrome codebase, but at a hunch I'd say there's been plenty of time to fix this. Maybe Google's Chrome team have other challenges to deal with, and making migrating back out of Google-space simple is just not a priority at the moment.
Google could have easily buried the article. As it is googling for "os x take google chrome out of full screen mode" brings the OSX Daily page as the first hit - without advertising (at least at time of writing) to obfuscate the message.
I still suspect Google's behaviour. Everyone is vying for the desktop at present. It's a natural survival of the fittest that goes beyond the desktop, to phones, to tablets, to apps, to your dreams. Making it harder for people to swap focus makes commercial sense, so neither the asymmetry in swapping modes nor the longevity of the problem surprises me.
More fun facts: Firefox also exhibits this behaviour. On OX Lion you have to hit Command+Shift+F to get back out of full-screen - though thankfully just the once. Only Safari drops back out on hitting the Esc key, which feels the most natural way of doing things to me.
Tin-foil hat theories are best left to another post.