Scenario: disaster splits the Internet into dozens unconnected fragments Major US cities restore connectivity locally for a couple of days, but the internet islands are separated (or have very poor interconnectivity).  Two days later, connectivity is restored resulting in, say 50 separate blockchains full of transactions.  NYC leads the pack with 5% of all transactions and is the dominant chain.  How do we reconcile 95% of the transactions (from 49 chains, less than 5% each) ?  Would anyone in their right mind use bitcoin in times of trouble?
Most merchants accepting Bitcoin transactions require six confirmations, so if there is no connectivity then, of course, there would not be any confirmations.  

So while continuous connectivity isn't required to use bitcoin in commerce, at least sporadic connectivity is.

For merchants who accept on 0/unconfirmed (e.g., retail), the rely on protection from the network to know of a race attack.  Without connectivity, that protection is gone and the merchant is vulnerable.  If the customer can be trusted, then a transaction on 0/unconfirmed can be accepted even without continuous connectivity.

For when extended outages occur, there wouldn't be multiple blockchains as there's no chance some isolated node (or region of connected nodes) would have the hashing capacity to make it to even one or two confirmations nonetheless six confirmations.  So there's no isolated forking issues to be concerned about.

Futher discussion on the suitability of bitcoin on an "disconnected island" occurs here:


http://bitcointalk.org/index.php?topic=106302.0