I'm sure what I'm about to say has probably been said many, many times in this thread already but without reviewing all the previous pages I'll just let you know what I've been doing since 2003:
DH & I can't go anywhere without our computers and at least 1 cellphone.
I am with Rogers (have been for years). And between DH & I we are in the U.S. combined for approx. 6-8 weeks throughout a year. What we did was we had 2 of our cell phone (that we got from Rogers)
unlocked by a third party that supplies you the code to unlock them, cost $15 p/p. These 2 phones then became our permanent travel phones. As we did "hardware" upgrades for our phones we just kept our old unlocked phones for travel.
Each time we go to the states, we stop in at an AT&T store and buy a pay-as-you-go SIM card, with a plan we usually go with $2 a day of use which is unlimited US calling, unlimited text, voicemail & call waiting. Long distance, calling to Canada is .25 cents a minute,free incoming calls from Canada.
http://www.att.com/shop/wireless/plans/prepaidplans.html
So when we need both phones (so we can call people within our group) we get 2x $25 prepaid card. expires in 30days if not reloaded.
In July, DD & I were in California for 2 weeks- 1 unlocked phone, but knowing that this year I would need a min of 5 weeks of cell phone within 90 days, I bought a $100 card this card will not expire until Jul. 2013 if i don't roll it over. There are other plans that will included data if that's what you need.
For me this is easier than doing "Tracfone"- it works exactly the same way (same plans etc.) but then I'm not locked to Tracfone (AT&T or Sprint) and I can use my unlocked phone in other places in the world...I can even continue to use in Canada if I want.
Bonus: if I save my contacts to the phone-not the SIM card I don't lose them or have to reload them with each tracphone.
Short summary:
1) Get your phone SIM phone "unlocked"- if your worried about doing this use an older phone that you might have. (e.g. I just got a iPhone, but it is my old Blackberry Pearl that I got unlocked)-not all carriers use SIM cards in their phones
2) Buy a prepaid phone plan with a SIM card included, when in US. (Walmart, BestBuy, AT&T store etc.)
3) Put the card in the phone & your all set to go!- No more Rogers roaming