Google Gears : Architecture for Offline Data
Google never ceases to amaze me what they make. Recently they have released the Google Gears framework for offline data storage for online applications. Now we all know from the Microsoft world, that a "Smart" client does exactly that, if you design and develop it as such.
I don't want to compare or contract Google gears with smart client technology, but rather expose how Google explains simply the need for a data access layer in all applications, it goes on to explain how to abstract the data access layers into a Server DAL and a Local DAL and utilizing a background synchronization process to synchronize the Local Database with the Internet Database.
Even if you don't plan to use Google Gears, I suggest reading up on the Google Gears Architecture.