For people moving around in Europe and other countries where Vodafone has roaming partners, there is a fairly cheap option with a prepaid plan of
Vodafone Irland for Vodafone Live as Pay as you go. It gets you 50 MB free traffic a day for 0.99 €/day. The 50 MB are enough for email a some web surfing.
FAQ:
How much does it cost to use mobile internet abroad?
Vodafone Mobile Internet is the same price when roaming abroad as at home. The service will be accessible from any country where we have a 3G or GPRS roaming agreement.
Vodafone Roaming Map:
http://www.vodafone.ie/planscosts/prepay/roaming/map/
So it should work in North America, too. But I have no confirmation yet. In South America there are no roaming partners for prepaid customers yet.
The prepaids are currently sold at
Ebay. I have one, and it works fine so far here in Germany, and confirmed by others in a lot of countries in Europe.
I think the link provided in the auction regarding the roaming countries is for a different product, as the FAQ of Vodafone Live says has no restrictions regarding the roaming. Probably someone has to test this.