Apple’s iPhone 3G apparently created a rather large migration: 30 percent of all the smartphone’s buyers bailed on their existing carriers and moved to AT&T in order to purchase the device, according to an NPD Group report released today.

AT&T has been the iPhone 3G’s exclusive carrier in the U.S. since the smartphone was released in June. According to the report, which covers June through August, 47 percent of those switching to AT&T to get the iPhone 3G came from Verizon Wireless, 24 percent from T-Mobile, and 19 percent from Sprint. This has to worry these carriers more than a little bit, especially Verizon.

“While the original iPhone also helped win customers for AT&T, the faster network speeds of the iPhone 3G (have) proven more appealing to customers that already had access to a 3G network,” sates Ross Rubin, NPD director of industry analysis.

By contrast, about 23 percent of consumers overall switched carriers from June through August (still a pretty big number).

During that same time frame, the smartphones with the highest unit sales were Apple’s iPhone 3G, followed by RIM’s BlackBerry Curve, RIM’s BlackBerry Pearl, and the Palm Centro, according to NPD. Palm has been a distant third in the race for awhile now.