Over at Fierce Wireless, Mike Dano reports on new numbers from the NPD group examining mobile data usage among carrier customers:

New data from NPD Connected Intelligence shows that Verizon Wireless’ iPhone users are the most prolific data downloaders among the nation’s top four wireless carriers. The firm found that the iPhone customers of Verizon used an average of 1.58 GB of cellular data per month in September.

The findings are noteworthy considering Verizon charges users on a per-MB basis, while rival Sprint Nextel has heavily advertised the fact that its iPhone plans offer unlimited data access. “Unlimit your iPhone,” proclaims Sprint on its website. “No metering. No throttling. No overages.”

According to NPD, AT&T customers average 1.35 GB each month — putting them just behind Verizon customers — while Sprint and T-Mobile customers use less than 1 GB a month.