I can tell you that Mobile Vikings is a prepayed card for everyone.
You'll have to pay for an entire month (€15), but that is non-recurring.
I think (but maybe Wouter cut you some better deal) you also have to pay
€15 for the sim card.

correct...

15 euro administrative costs for the sim card which is not changed into "voice/sms/internet" services... 

so they would be spending 30 euro's :(

there is a solution...

they have a 12 euro data-only sim card solution

the 2gigabyte will be only for 1 month (no grace)

it will cost 15 euro but you don't need to "recharge" it with 12 euro to get the data-only... you can use it right away to surf the net.

and don't tell me all the fosdem visitors require voice/sms ? They can use whatsapp, jaxtrsms, skype, voipster, voipbuster or other sip & smsclients :-)