Here's news on an interesting offer recently started by German MVNOs using the e-Plus network (Simyo, Blau, Alditalk): Since October 1st, their prepaid SIM cards are now activated for data roaming in the EU and a number of other countries. Price per 100kb block is 0.19 euros. While not exactly cheap, it's a 10th of their previous price and a 10th of the price of most other operators around Europe.
I wouldn't use it with a notebook but for small screen web browsing and mobile e-mail reception it sound quite affordable. In the countries they support data roaming outside the EU (btw. Switzerland is part of that list…) the price per 100 kb block is 49 cents. For details, see the Prepaid Wireless Internet Access Wiki.
Recently, I tried the offer with my Alditalk prepaid SIM while in France. The first time I tried, right after the start of their offer, 49 cents per 100 kb block were charged. I sent an eMail to their hotline, asking what was going on and they admited that they have a problem with their billing system and returned the money that they overchaged. When I tried again last week, the billing system was fixed and the proper amount was charged.