Does this always have to be turned on or just when wifi or 3G is on?
To sync the device, unless connected to the PC, you would need an Internet connection on the device, be that a wifi or 3G connection. Sync settings are automatic on the device, but can be changed in a variety of ways.
Copy and paste the link below into your browser's address bar for more information.
Sync settings Page 111-121.
http://cache.vzw.com/multimedia/mim/htc_incredible/incredible.pdf
