This is interesting! As a form of volcano relief Lonely Planet is offering a bunch of their city iPhone guides for free (they normally start around $16) through April 22nd (that’s in two days, so hurry!). This includes the Paris guide I used during our recent trip, and has an offline location aware map, which is fantastic to use while avoiding roaming charges. I’m not sure if all the city guides have them, but I would expect they do.
I found this news at David Lebovitz, who is offering his suggestions on 10 Things to Do If You’re Stuck in Paris. If you are stuck in Paris you have both my sympathy and my envy.