It's fascinating to see how new ideas are emerging in different countries to market prepaid SIMs for Mobile Broadband Internet access. The latest and greatest I've seen to date comes from Polish operator Plus. Not only do they offer prepaid mobile Internet access, they even have English instructions on the sales package to make it easy for inbound roamers to use their service. I haven't seen that anywhere else yet. Details can be found here. Thanks to Chris_M for putting the information into the Wiki!