There's an interesting article in the German magazine Spiegel about the length of road networks in the world:
http://www.spiegel.de/auto/aktuell/0,1518,573827,00.html
(I don't think it's available in English (although they have English pages) but you can probably figure most out even if you don't understand German.)

Seems the biggest chunk of OSM road data is already there thanks to the TIGER import.
And we're not too bad off in Germany, only a meagre 231500 km of roads to map and quite a few active people.
Once we're done and don't even have any farm and forest tracks or hiking paths left, we can all move to India or China and have work there for the rest of our lives. ;-) But I think I'll rather go to Brazil and map the Copacabana really well. ;-)
(Actually I'm astonished there is data of China at all. Last I heard was that the regime there didn't want geo data to leave the country.)

P.S.: There's another article of the "10 most..." series about cost of parking space. Also very interesting. I thought Manhattan would be more to the top of the list... http://www.spiegel.de/auto/aktuell/0,1518,566264,00.html

So long,
Markus