My Vodafone data sub has been infintely more useful and flexible since I bought a MiFi personal wifi hub last year. MiFi is small, about the same size as a small cellphone and accepts any 3G data SIM. As soon as the on button is pressed the unit logs on to your service provider and opens a personal network for up to five devices. So your MacBook, your iPhone and (soon) your iPad are all connected to the world in seconds. If you happen to be travelling with friends or family you can let them share the broadband.
A MacOldie refinement is CarFi: a mini wifi network in the car. My vehicle has a second power socket in the luggage compartment. I recently had it rewired so that it is available even when the ignition is off. The default for obvious reasons is to have the socket off when you leave the car.
Using a handy Belkin USB plug, I connect the MiFi and leave it running in the back.
So now I have wifi wherever I go without need for wires, dongles or other peripherals. It's a neat and trouble-free solution. The power needed to keep the MiFi topped up is probably so low that it doesn't matter if the unit is left plugged in while the car is left for reasonable periods. You need to be sensible about this, however. The method also ensures that the MiFi unit has a charged battery if you need to remove it from the car. One of the snags with the MiFi is the low battery life up not more than four hours, so it makes sense to keep it topped up when you have access to power.
What you need: A MiFi unit or similar 3G mini router. The MiFi is made by Novatel and can be found in the UK for just over £200. The Belkin Micro USB car charger is a neat plug for a car power socket (£2.99 from Planet Gizmo). All you need then is a subscription or pay-as-you-go data card. I recommend the Vodafone network because it is fast and has good availability in the UK. Expect to pay from £15 per month (for 3GB). Alternatively, try T-Mobile's £2 per day deal if you are not a regular user. This £2, bought on a top-up basis, gives unlimited data for one day. So, unless you expect to use the data service on more than seven days a month it could be the best for you.