If you travel a lot, and you haven't seen it before, I'd heartily recommend checking out the prepaid wireless data resource here.
It lists prepaid data SIMs and tariffs in numerous countries - invaluable if you want to avoid ridiculous and unjustifiable mobile broadband data roaming prices.
Thanks to the Wiki, I managed to get hold of a Yoigo prepay SIM in Barcelona, and it gives me flatrate (albeit slowish) mobile data for €1.20 a day. The SIM costs €10 and comes with €10 of credit. It's great for email and web access on my phone, plus I'm finally able to use Google Maps and navigate myself in a foreign city, for less than the price of an equivalent taxi ride.
It was a bit of a pain to find and buy the SIM. I needed to take my passport as ID, and the sign-up process took about 15 minutes in the store. In other countries like Portugal, the process is a lot smoother.
It's a very good advert for the benefits of unlocked, unsubsidised smartphones. It's also highlighted that I need to get myself an unlocked or reflashed 3G dongle.
Unfortunately, there doesn't appear to be an equivalent provider in the US - even on just with flatrate EDGE would be useful.