I use a T-Mobile and Google Voice solution when I travel to the States.

My buddy gave me his T-Mobile sim from a pay and talk phone he had purchased from Walmart. Once I set it up online and had a US number, I was able register on Google Voice.

I use the $3 a day plan for calling within the US and for data (which also works fine for tethering your tablet). I use Google voice to call & SMS back to Canada with no additional charges (other than within the $3/day).