Apple stores will soon be able to connect a newly purchased (full price) iPhone to a pre-paid or month-to-month plan right in the store. Citing unanmed sources 9to5Mac reported that Apple is adding the options in order to boost iPhone sales.
Currently a full-price unlocked iPhone can only be tied to a plan through a carrier store or a separately purchased SIM card. This means there's another avenue for purchasing from the Apple Store, presumably giving Apple a bigger piece of iPhone sales.
This is likely to start in the last week of June, featuring AT&T's GoPhone kits and T-Mobile prepaid SIM cards. Store employees will be promoting T-Mobile's US $50/mo (with 1GB LTE data) and $70/mo (5GB LTE data) for T-Mobile customers. Those wanting an AT&T plan will be offered a $60/mo pre-paid plan (with 2.5GB of data and unlimited talk/text).
Keep in mind you have to mean it; these activations will happen in-store and devices can't be returned for a refund.