You might not know, but I think you are talking about the right thing!
I can't say for the US as I am in the UK, but here I have a sim for my iPad. It is supplied by 3 (a mobile phone company) works on 3G and has 3GB of data and is valid for three months from first use. They do cards for one month and twelve months too and this one suits me best. You can use it at whatever rate suits you. As you are seeking, I use it when I am not using free hotel wifi etc.
You probably need to check the prepaid sim thread for the USA for more details.