Fly to the USA with your iPad 3G. When you arrive go to an Apple store and tell them that you are there on holiday and that you are looking for a 3G card. They will GIVE you an AT&T micro SIM card for free.
Note: If you can't find an Apple store you can get an AT&T card from an AT&T store but they will charge you $15.00 for it.
Once you have the card find a free wifi network (obviously the Apple store if you are already there, but any Starbucks or McDonalds or many cafes will do. I used the cafe route because I didn't have my Ipad with me when I went into the Apple Store. Insert the SIM card, join the wifi network then go to AT&T's web site and find the page about Ipad plans it's quite easy to find but not screamingly obvious, then sign up for a plan.
There are two basic plans available. 250MB for 30 days = $15.00 or 2GB for 30 days = $25.00. I went to 2GB route and gave my iPad a fairly heafty thrashing over a 12 day period including using it as a GPS in the rental car, Skype and some video viewing and managed to chop through a bit over 500MB so I think that the $25.00 plan is the better value for all but basic use.
You will need a credit card and an email address to sign up. Hotmail is fine and so is an overseas credit card but there is a catch. The plans are monthly if you are going to be in the USA less than 30 days you MUST cancel the plan either soon after you sign up (it will still run the 30 days) or immediately BEFORE you leave the country otherwise you will get another 30 days worth of data billed to you next month.
Coverage? I used mine in northern California from about Carmel through San Francisco and up into northern Marin county and generally the coverage was pretty reliable and consistent. I don't know what it would be be like in other less populated areas.
When you get home take the AT&T SIM out and put your home country SIM back in and you will be back on your home 3G network without any problems.