Solar power mobile phone station
Hook up with solar enthusiasts, or a solar company, to provide solar powered charging stations. These could be sponsored by any company or by a mobile phone network retailer. Make sure there are lots of different phone power adaptors available.
Pedal Power phone charging
Get people on a bike and make them work for their networking. Pedal power systems are available to power stages and cinemas, but they are also great as single bike power stations, to recharge mobile phones and camera batteries. A great interactive 'exercise' for many types of events.