In hopes this will be useful to others:
I have an unlocked AT&T phone (Samsung Galaxy Nexus), and needed a data plan in Belgium (Brussels, specifically). I really don’t intend to text or talk, but data is important. Most of the news kiosks are happy to sell you a SIM, but with no data portion.
A fellow Mozillian, Ben Kero, recommended BASE wireless. There was a spot in the airport, but they were out of SIMs. I borrowed some wifi, looked up another location on base.be, and went there.
They have a 1-GB plan for 15€, called “surf & mail 15”. What you’ll get is a 15€ SIM card with 15€ credit on it, and a brochure giving instructions to send a text message to activate the 15€ plan using that credit. When I did so, I immediately received a text indicating I did not have sufficient credit, but the data seems to work.
Which is good, because the wifi in the apartment we’re staying at is pretty poor.