I live in China, and prepay is great. I still pay full price for my phones, but I come out ahead in the end.
As far as I can see there are no pay as you go data plans in the US. Only subscription plans. If there are I'd like to know of them
I found out that AT&T has annoying restrictions based on the IMEI number - for a IMEI that maps to a computer, they will only offer you the $50/month plan. For a IMEI that maps to a tablet, they will allow you to to buy the $15/month plan. FWIIW, they require you to enter a IMEA number, not the number of the device you are using the SIM in. ;-)
I am currently running my phone on a T-Mobile on a $30/month pre-paid plan[2] with unlimited (for some definition of "unlimited") data and 100 minutes of talk. Unfortunately, they have poor coverage in about half the area I frequent... I end up roaming on AT&T and don't get data coverage there.
Both those options are essentially the same as your Singapore plan.
See footnote 6 on the page you linked to, then click on any of the plans, then click View Details next to any of them.
I'm visiting New Zealand now. The first thing I did when I got off the plane was purchase a 3G prepaid (2GB) USB dongle for $70, and I can top it up if I need to and move the sim card to a phone if I want to. I didn't have to sign or agree to purchase anything extra.
I can do this in Australia, too. What the hell, USA?
https://buyasession.att.com/sbd/Common/ShopRatePlans.action
Note that the plan you really want is the "Tablets and Gaming Devices". The "Computers, Netbooks, Aircards, Others" plan is a rip-off except for the $50/month 5GB plan, which is the same as the "Tablet" plan. On the "Computers" lower data rate plans, it is $15 per day, $30 per week. Data is data, so they are just ripping you off.
AT&T DataConnect Pass plans
* No Long-Term Contract
* Plans are active for 30 days or until depleted.
* Plans automatically renew every 30 days for domestic data plans, unless you cancel.
* International plans do not automatically renew.
* Plans can be cancelled at any time.
Domestic DataConnect Pass Plans Cost
DataConnect Month Pass 250MB $14.99
DataConnect Month Pass 3GB $30.00
DataConnect Month Pass 5GB $50.00It's such a good deal that even with carrier penalties for breaking contract it is still much cheaper over the course of a year.
\* I understand there's a soft cap on the data of about 2GB a month, but I haven't run into it yet.