Unless that software contains copy protection techonology, then it's not applicable to the DMCA.
Additionally, the Library of Congress has made an exemption for the prohibition of circumventing "Computer programs in the form of firmware that enable wireless telephone handsets to connect to a wireless telephone communication network, when circumvention is accomplished for the sole purpose of lawfully connecting to a wireless telephone communication network.*
It may be debated whether nor not using an iPhone to connect to a network outside of AT&T is lawful, but if you're paying for the service of that other vendor, I would think that it is indeed lawful.
*Rulemaking on Exemptions from Prohibition on Circumvention of Technological Measures that Control Access to Copyrighted Works