I know this is an old thread, but thought I'd update it.
I now have a permanent T Mobile phone number in the US after buying a sim card at a T Mobile store (Rosecrans at Manhattan Beach).
I've used it on 5 trips to the US so far, and as long as my balance is over US$50, it remains active.
I have a message on it that if I'm not in the US, then call me on my australian number, and when I go away, I put a message on my australian phone to tell them to call me on the US number.
When I take off from Oz, I swap sim cards over, and when I leave the US, I put my Telstra sim back in.
Works really well, and if I want to call home, I use Skpe from the hotel where ever I'm staying, and with free internet being almost everywhere in LOTFAP, it means call costs are very cheap.