In Italy this is the default (pay as you go). There are maybe something like 1% of mobile phone users using a contract.
IMHO the best data provider is WIND: there is a plan where with 9 euro/month (called "Internet NO STOP" http://www.wind.it/it/internetmobile/scheda542.phtml) you have 1 GB of traffic in GRPS/EDGE/3G/HSUPA, when you consumed the whole Gigabyte you can still surf without being charged but with limited speed of 32 Kb/s.
There is another similar option with 19 euro/month where the limit for the fast speed is instead 10 GB.
The SIM lock is why I have yet to buy an iPhone. I don't mind the mandatory contract and I don't mind having to buy prepaid SIM cards when I travel abroad. But I live in Canada and travel elsewhere quite often; roaming charges are such a blatant rip-off that they render the phone useless when it is most needed.
Yes I know I can jailbreak it, but I'm not going to buy a premium product to only be saddled with sysadmining it so i stays up-to-date.
Then you can stick whatever SIM card you want and it'll work. Unfortunately, you'll be stuck with paying a lot at the beginning for the phone. It's also possible that you can get your carrier to unlock it; I know some UK carriers will.
This is a cracking resource if your travelling abroad, one thing to mention is that if your travelling to the UK, 02 has a £2 a day for 500meg on both the iPad (microsim) and datastick format, however it usually lasts more than 24 hours as they seem to have a script that runs at midnight cutting anyone off that has been on for 24 hours... if you do it early in the morning you tend to get 36-40 hours out of it..
I would recommend T-Mobile. I'm using a Blackberry Curve 8520 using their unlimited data plan, 500minute unlimited text plan. $70.71/month to month/pay as you go plan.
The EDGE service is spotty, like Boost but they also have sufficient selection of 3G and Android phones.
I am in a unique situation. I am using a GSM phone with GPS mainly as a location sensor. So, I don't need any voice minutes, just unlimited data, for as low a cost as possible.
3G would be nice, but don't even really need 3G.
Other than spotty coverage, Boost fits the bill perfectly. They have unlimited data for 0.35c per day.
Most of the information in the Boost Mobile section is completely wrong. They run on Motorola's iDEN network primarily although some of their recent phones run on Sprint CDMA. Those CDMA phones do support 3G as part of their Power Vision network. CDMA does not use SIM cards, iDEN does but they are by no means GSM.
Did you miss the part where the page also says Android?
I am using a Motorola i1 running android with a sim card in it.
If you are traveling to the US, one of the cheapest ways of getting a data plan without a contract is to buy a prepaid Motorola i335 for $30 - $50 with a prepaid connection and tether it for 35c / day.
The point of the site is to find out where to purchase a prepaid GSM SIM to use in an existing phone. Not where to buy all new equipment on an unreliable service.
Are you sure that Boost mobile works for iPhone? Their coverage seems to be limited to iDEN and CDMA, neither of which is compatible with iPhone's GSM requirement.
IMHO the best data provider is WIND: there is a plan where with 9 euro/month (called "Internet NO STOP" http://www.wind.it/it/internetmobile/scheda542.phtml) you have 1 GB of traffic in GRPS/EDGE/3G/HSUPA, when you consumed the whole Gigabyte you can still surf without being charged but with limited speed of 32 Kb/s.
There is another similar option with 19 euro/month where the limit for the fast speed is instead 10 GB.