Netherlands

Prepaid data
You can get a prepaid SIM for € 7.50 with € 2.50 calling time extra for free, for a total of € 10.

There is also a special offer with Vodfaone if you register your name/address you receive € 5 credit free to be used however you like (e.g. internet/call/sms blox)

You can then add a so-called 'Carefree Internet' Blox to this sim for € 9.50 for 1 month unlimited(*) Internet on your cell phone.

If you recharge with the Carefree blox more than once in a 30 day period, the recharge amount is halved to € 4.25. it appears you can recharge as many times as needed in a 30 day period for this half price rate.

If you try to use this plan with an iPad, there are a few obstacles you'll face. Vodaphone sends you instructions and discount offers via SMS, so you won't get them. Also, you have to use SMS to enable BloX, which means you have to put the SIM in a phone.

See APN settings for your phone

Fair use policy
There is a 'fair use' clause that limits the "monthly data consumption to up to 10 times the average consumption of all users (excluding the consumption of the fautive contractant.)"[circular reference exception? ;-)] In practice it seems to be somewhere around 500 MB.

Availability
Vodafone SIMs can also be purchased in their own shops (enter postal code or name of city)

iPad Data Bundles
500 MB bundle € 20

2 GB bundle € 30

More info(link in Dutch)

Data is also usable in other (European?) countries than The Netherlands where Vodafone has a network.

You can cancel after one month.

Data rate max 3.6 Mbps (500 MB) or max 7.2 Mbps (2 GB)

SIM sizes
Mini SIMs availability unknown. Micro SIM available for iPad.

Tethering
Tethering is "allowed" on vodafone netherlands, i have downloaded over 6gb of data via 3g mobile PAYG SIM without problems.