How strange that this thread has resurfaced as we are thinking of going to Corsica again this year - 1st 2 weeks of Sept.
Just to recap we didn't book in 2010 and had no problems, the friends we went with went back last year without booking and had no problems (August), so we won't book this year.
It's worth noting, however, that we prefer quiet farm type campsites we don't need a bar, restaurant or swimming pool and we're happy away from the coast.
We were going through our 2010 photos the other night and I realised that I didn't post them - I shall try again over the next couple of weeks.
Cheers