Do I need a data plan with an unlocked phone that is NOT branded with AT&T?

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http://androidforums.com – Ok, I've been researching for the past 2 hours about unlocked phones and data plans. There may already be a post on here, but I probably read it and didn't have my question answered. So here's the deal: I'm currently on an AT&T family plan, without data. I do NOT want to add a data plan, but want to use WiFi. I am looking at an unlocked+unbranded Sony Xperia P. I really want this phone (I have an HTC Freestyle and it's total crap). I've been reading, like i said, about this topic, and there are literally hundreds of mixed answers. I need a straight-forward answer for this. I (General)