Not really an answer to your question but do you have a 3g phone? On holiday recently we stayed at a place with no wifi and I just set my phone (HTC Desire) up as a wifi hotspot. That way I could connect both my Tablet S and my wife's laptop to it through the normal wifi on both. Given a half-decent phone signal it was pretty quick too. Just a thought and it saves buying a separate 3g dongle.
You cannot use a USB dongle. What you need is something like the the Three MiFi device that allows you to connect over wireless. I have one of these and it works very well - you can connect multiple devices at the same time although performance will decrease as more devices are added dependant on what you are doing obviously.
I can use the phone as a hotspot but have only a 1 gig allowance plus the battery on the phone seems to bbe eaten up quickly. was hoping a dongle may be the answer will have to purchase extra mb's from Orange when I get out in the camper