Except for the special, desperate case where you would want to share that 3G connection between several Wifi clients, I don't see the advantages for individual use. It's easier and simpler to have a Bluetooth 3G phone and connect the tablet directly through it : one less battery to keep charged, one less item to carry. Plus the tablets last much longer using Bluetooth dialup than Wifi, in my experience. The use case for those pocket ethernet/Wifi routers you mentioned look much clearer to me... provided the LAN you plug them into has DHCP enabled (wouldn't work at my job :-).