AT&T's for the iPhone all include "unlimited data" -- that is, unlimited Internet usage. But only the premium US$130/month Unlimited plan allows iPhone users to make all the phone calls they want for a fixed price.

Skype for iPhone means anyone willing to figure out the app could potentially order AT&T's entry-level $70-per-month plan, then use Skype and Wi-Fi connections to make as many calls as they want to over the Internet, potentially cutting their monthly payments to AT&T in half (the AT&T Nation plan costs $130 per month for unlimited U.S. calls).

But Skype for iPhone will require an available Wi-Fi connection, which severely restricts its availability to users on the go, compared to making calls over AT&T and other carriers' 3G and EDGE networks.

Skype, an Internet telephone service launched in 2003 and acquired by eBay in 2005 for an eye-popping $2.6 billion, is popular among tech workers, students and others willing to deal with setup and configuration issues. Like any phone service provider, Skype has been criticized for . But users willing to tinker a bit are able to make free calls to other Skype users. They can call non-Skype phones in the U.S., Europe, China and most of the world for about , or buy for as low as $3 per month.

But adding a Skype app to the iPhone means even non-technical users can figure out how to get unlimited calling much cheaper than AT&T's price. New customers can always start with the lowest price plan and, if they're not happy with Skype, upgrade.