Well, when you add a file to your computer’s Dropbox, the file is then synced with Dropbox servers. Dropbox will then initiate the syncing process as soon as it determines a change has been made to the file. All linked computers and shared folders will then download any new version of the file. With LAN syncing, Dropbox will look for the new file on your Local Area Network first, bypassing the need to download the file from Dropbox servers, thus speeding up the syncing process considerably.

via Dropbox – What is LAN sync? – Simplify your life.

I’m not quite sure if this is a good idea.

From: http://tools.cisco.com/security/center/viewAlert.x?alertId=23896

When synchronizing user data and communicating with the host service, a Dropbox client normally uses TCP port 443.  In addition, the application may also use UDP and TCP port 17500 for communication via the Dropbox LanSync Protocol.

And…

Administrators are advised to implement an intrusion prevention system (IPS) or intrusion detection system (IDS) to help detect and prevent attacks that could detect the presence of Dropbox in a network.