So in the line of business application space,  I often need to acquire external inputs from real world devices,  such as weigh scales,   pressure and moisture sensors.

I've often use WebService calls directly from the mobile device to call services on P.C's hooked up to measuring equipment usually via a serial port.

However increasingly I use insert triggers on SQL tables to fire off requests to physical devices (using managed code) stored procedures.   So, the action of syncing SQL Compact causes results from devices to be captured.    This technique works really well and ensures that SQL Compact syncrhonisation is the only data exchange from the mobile device.   This keeps things simple (always good).     The only downside is that when standing next to a weigh scale etc. with a mobile device you have to be in range of the wireless network.   

So upshot of all of this,  is that our end user can wander around taking weight's and measurements almost 'magically' with no physical connection from the mobile device they are using...