Techniques of database slicing

I recently had presented to me the interesting problem (which I am still faceing) of finding a solution to the following: The client wants to have a central database with agents in the field being able to access the database. (No problem - Web technologies can handle this). In addition the client wants a database on each agents laptop that will hold all the relevent data for his area. The agent must be able to amend, add the data on his laptop should the internet be unavailable and on the next session the two databases must sync.
I am still working on this one but if you can share any knowledge... that would be great.