Public Mobile doesn't offer service outside of Canada.
If your phone isn't unlocked already you could unlock it by following the instructions posted here, or by going through a third party.
Once the phone is unlocked you could find a compatible carrier stateside and purchase services with them under an american phone number for the duration of your trip.
Lachman, I recommend the roammobility.com solution.
I used it in Chicago last year and it worked well. I had to buy a SIM card (in Canada) and then pay for the days that I want to use my phone in the US. For five days I paid $3.00 per day, but if you spend longer periods of time in the US then the price per day goes down. (That was much better than the $30 for 100 minutes that I paid for US roaming with my previous cell phne service.)
With the roammobility.com SIM card you will get a US phone number and unlimited talk time. You can also order data, at an extra fee, if you wish to. I just swapped SIM cards as I crossed the border.