It wasn't as easy as I had hoped.
The T-Mobile one when inserted was supposed to have an activation code sent to the phone. It never happened so I had to call their call centre. After being transferred around many times and facing the IVR's from hell I finally found their pre-paid sales department. They asked for the SIM#, phone IMEI and gave me an activation code. I then was transferred to sales who were able to activate the SIM and tell me my phone number (I chose the area code).
I chose a plan where for one month for $30(US) I get 1500 minutes of calls and local SMS and 30MB of data. Should be enough for me since I am only here for 18 days. Once the phone was activated I was able to call 611 and pay the $30 for the plan (activation gave me $3 of call time).
Then the AT&T one. Again when inserted into the phone it came up with a SIM error. I called AT&T and again after some transferring I got to somebody who could help. Again it was a bit tedious as I read out the SIM # and then again they wanted the IMEI of the phone I was planning to use it on. I gave that but also asked if I could use the SIM in another phone and they said I could. This one was for my daughter so I chose the plan where if she doesn't use the phone on any day (either to make or receive calls or SMS) there is no charge but if she makes a call or sends a SMS, then she is charged $2 but for the rest of the day all calls and texts are free. There is no data on this plan since she doesn't need it.
Finally thought I would get one for my wife's iPhone4. Went into the mall to the AT&T store but it was so busy that I left and saw a T-Mobile booth in the store. Asked them if I could purchase a prepaid SIM from them and they said sure. A ton of plans available and a special deal on for the month so I chose what is usually priced for US$60/month but $30 for this month a plan that has
1000 minutes to any phone
unlimited minutes to any T-Mobile phone and unlimited calls to any other phones evenings and weekends
unlimited US SMS
Seemed like a good plan.
This was put into the Phone4 (I had brought along my handy SIM cutter :-) ) and works fine.
Now they didn't charge me at the time saying they would send a bill to my US address so I don't know what would happen if you didn't have one. They also asked for my Social Security Number (which I have) and ID (I presented a California driver's licence) so again I don't know what would happen if you didn't have those. Possibly they would take the payment there and then?
Anyway the information might be useful to travellers to the US.
The T-Mobile plan I got for the iPhone4 seems to be the best. If the specials are not on, then for the tourist I think a $39(US) plan for 500 minutes and unlimited SMS is a good option. Beats roaming costs. Then use something like Google Voice to make international calls.
It was too good to be true. While the plan is valid, it's designed for a US resident who wants a monthly plan without a long-term commitment. So there was an additional $35 activation fee which I saw upon reading the fine print. So I went into a T-Mobile corporate store and complained. They told me that the best plan for me was a $30/month plan with $10 for a SIM (which they waived) and it gave me 1500 minutes of calls/US SMS and 30MB of data. I then addded $10 to that for international SMS (US$0.35/SMS). They also cut the SIM for me on the spot. Of course this way you don't need ID since the plan is prepaid.
When their call centre opens up again I will call in and cancel the first SIM card and plan.
So the advice I can give is, go to a T-Mobile corporate store/booth and insist they waive the SIM card charge. They appear to do that if you ask. Then choose the plan you want.
If you want to make calls back home then setup a GV number, add some money to it and call using GV since they cost only US$0.02/minute to landline and $0.22 to cells. T-Mobile charge way more for calling international.