T-Mobile prepaid will begin offering a $10 one week unlimited talk and text pass starting June 28.
The talk and text pass will be an add-on to T-Mobile's $3/month pay as you go plan. It will work the same way as the $3 plan's currently available data add-ons: 5 for 500MB good for one day or $10 for 1GB good for 7 days.
The pay as you go plan provides 30 "use them or lose them" voice minutes and texts per month. Additional texts and minutes are 10¢ each.
While the unlimited talk and text pass combined with the 1 GB data pass costs just $20, a customer new to T-Mobile would actually have to pay $38 to walk out of a T-Mobile store with a SIM good for a week's worth of unlimited talk and text and 1 GB of data.
AT&T GoPhone prepaid looks like a better short term deal; $2/day for unlimited talk and text, including international texts, plus $1/day for 100 MB of data per day. AT&T SIMs list at $10 each but in my experience AT&T stores will generally provide a free SIM when you purchase airtime. AT&T airtime comes in $15, $25, $50, $75 and $100 increments so a visitor could get 8 days of service for a total outlay of just $25.
Updated: US Prepaid GSM Voice, Text and Data Options For International Visitors
Prepaid Operator Profile: T-Mobile
Prepaid Operator Profile: AT&T GoPhone