I'm on H20 wireless on my iPhone 4S and it will work the same. Activate the sim before inserting it and then once you insert the sim, it should work right away. No unlocked iPhone needed because it is on At&T. Now that H20 has once again lowered their data plans from 2GB to 1GB, I'm jumping to Red Pocket!!! Heard they are WAY better in Customer service & plans.
I just bought a Red Pocket SIM Card on Amazon. Switching from h2O to RedPocket as soon as I can. H2O is now making the $60 only 1GB of data.