The iPhone 5 just came out and people are wondering how to place it on Straight Talk. You can use the Noosy Nano Sim card cutter to get your regular/micro sim to nano size. Some of the cutters actually include sandpaper which will help you get the sim card to the right thickness.
Straight Talk doesn't currently offer nano sim cards on their site. I think Straight Talk will carry them next year. Nano sims are used in far to few phones right now for them to offer it on ST. By sometime next year more and more phones will start to use the new sim which will prompt ST to offer them.