We are implementing a system in the company I work for where by we will need to install the system in various sites of the same client (warehouses). The users in all sites should see the same information. The system should be able to work in each site when the network is down. What design architecture solution would be most suitable?
I suggest you consider CouchDB. Its robust replication feature is designed specifically for this sort of use case. It supports both continuous replication, which could keep the data in the various warehouses in sync in near-real-time during normal operation, and occasional replication, which could be used to sync data after a network outage.
There's a really good free O'Reilly book: CouchDB: The Definitive Guide, which has a chapter on replication.