Sprint has released an unlimited mobile-to-mobile plan just one day after AT&T announced its own unlimited calling plan.

Sprint’s new $70-per-month plan adds unlimited mobile-to-mobile calling to its existing all-you-can-eat data plan. AT&T’s plan will debut on Sept. 20. It allows customers with an individual $60-per-month Nation plan unlimited inbound and outbound calls to five domestic phone numbers.

Both Sprint’s and AT&T’s plans are time- and network-agnostic, but AT&T’s plan also covers landline numbers. AT&T’s plan targets voice-centric consumers by incorporating roll-over minutes with unlimited calling to select numbers, while Sprint is targeting heavy data consumers.

Sprint and AT&T each carry rival handsets: the Palm Pre and Apple’s iPhone, respectively. Though the Pre was touted as an iPhone killer prior to its release, it has failed to garner the same following, currently selling an estimated 30,000 per week.

Yesterday, Sprint upped the ante against the iPhone by cutting the price of the Palm Pre to $149.99 with rebates and a two-year Sprint agreement. An internal communication error put the Pre at around $100 for a brief period of time before the mistake was caught.

Sprint also said yesterday that it would be carrying the webOS-based Palm Pixi, a messaging- and media-friendly smartphone with a qwerty keyboard and limited-edition covers.

Sprint and AT&T are both competing for high-end customers in an increasingly saturated wireless market. The saturation already has led to heated competition in the prepaid space, where unlimited voice and data plans have become prolific.