Once again, guys, thanks for all of your inputs!
You guys are missing the point...
1. He is from Europe where everyone OWNS their un-locked GSM phone...Initial purchase cost is higher, but monthly costs are generally lower in Europe than the U.S. Most users are free to change providers at any time...just put a new SIM card in the phone and go. In the U.S. we are used to free or low cost phones due to the phones being locked-in to a single carrier; and we pay higher montly billing charges which include fees to pay for that "free" locked-to-a-single-provider phone.
2. He just wants to get a Pre-Paid SIM card to plug into his phone for a short period of use.
3. Most of the phones he could buy in the U.S. would be useless to him in Europe, as they are locked to U.S. carriers. A waste of money to buy another phone.
This sums it up just right, maybe I wasn't much clear; yep, in Europe we're used to low fees and buying unlocked phones... for what I saw, fees in US are pretty higher than we're used to.
T-Mobile because you can use buy a simcard, put it in your phone and put however much cash you want on it.
To get a bit technical, this is why you want ATT. Your phone is capable of running HSPA 14.4 Data in the GSM 850 and 1900 mhz bands. This alligns with ATT's spectrum and data network to allow you to receive 3G data speeds.
Essentally someone can argue forever about what is best, but the above written WILL WORK for what you have described in terms of primarily Data phone to be used temporarily, and at a reasonable price.
Unfortunately T-mobile won't work for me 'cause, as Mk84 pointed out, my phone works on different frequencies than the one used by T-mobile (in fact, T-mobile had LG manifacture a specific version of the Optimus 2x, called G2x in US, working on the 1700 MHz band for data)
What phone do you have in Italy?
LG Optimus 2x, quad-band Android smartphone working on the 850/900/1800/1900 MHz band.
<snip> What I would do is buy the cheapest ATT GoPhone (thats the brand) and activate it at the previously mentioned plan. <snip>
Then shove the simcard into your Optimus 2 and you're good to go.
For what I see on AT&T and other carriers websites, it seems not every plan can be activated for smartphone (you have to buy the more expensive ones, of course!), I assume if I buy phone and activate one of those plans, it won't work when I plug the SIM into my phone, am I right?
OT Wurz, Rap, good to see you guys again!

I'd like to come back to AH, but when I checked out the game again, in March, it was so alike to when I left... there's need for change (or at least for me to take a loooong break!). See you soon! /OT