Put an iPhone 4s on Straight Talk?

I'm planning on buying an iPhone 4s for AT&T. I want to change it to Straight Talk because of the cheap plan. If I never activate the phone when I get it, would I have to pay an Early Termination Fee? If not, then what are ways to get around it?

You can do this, but will have to pay full retail price for the iPhone 4s ($549)

I would not buy "for at&t" to make sure you dont have a problem. Although the ads I seen said 4 and 5, not sure why they didn't say 4S but something worth looking at.