In Slovenia, you can buy a data-only sim card from Mobitel.si for 15 euro; this comes with 24 hours of unlimited data transfer. Then, you can top it up with increments of 5 euro (1 day), 10 euro (3 days) or 20 euro (10 days). See here:
Mobitel.si
They have both regular sim cards and micro-sim (for iPhone 4).
Shuly