Yes, you can use the iPhone 3GS with Straight Talk. To get 3G speeds, make sure to order the AT&T compatible regular SIM. If you choose the T-Mobile SIM, you'll be limited to EDGE speeds as the iPhone doesn't support T-Mobile's 3G frequencies. It's $14.99 for the SIM and $45 a month for the service. You'll have to change the data APN, which you can do from http://unlockit.co.nz from your iPhone on Safari on Wifi. Choose 'United States' and 'Straight Talk'.
Straight Talk unlimited everything plan for $45