Members of Alltel Wireless’s My Circle plan can now get unlimited text, video and picture messaging to anyone in their “circle” as well as other Alltel customers. In the latest addition to its service offering, the nation’s fifth-largest mobile carrier today announced any subscriber signed up for any messaging plan costing $7.99 per month or more will get unlimited SMS and MMS.

The service is likely to vastly broaden the appeal of the MyCircle plan as more customers—particularly younger ones—shift more and more of their communications from standard voice calls to text messaging. The move, however, also has the potential to further dilute the value of SMS, carriers’ primary source of data revenue. Carriers have been offering bigger and bigger buckets of messages for the same price over the last few years, and most of them, including Alltel, offer unlimited messaging plans of some sort. But the plans tend to be in the $20-per-month range or higher. The new MyCircle add-on will essentially give customers unlimited SMS and MMS to customers at sub-$10 prices, albeit to a limited audience.

Alltel first expanded the flexibility of My Circle in February by letting consumers choose to move from 10 circle members, the standard since 2006, down to five or up to 20. The plans, which start at $50 per month, already include unlimited free calls to and from any combination of home phone, office or wireless numbers anywhere in the US. The program is similar to T-Mobile’s myFaves feature, which was introduced six months prior to Alltel’s social-network-inspired program. To date, T-Mobile does not offer customers unlimited messaging amongst their 10 phone numbers in the myFaves program.

In preparation for its impending take over by third-largest carrier Verizon Wireless, Alltel has been upgrading many of its service offerings of late – and its network. Just last week, Alltel said it would begin to upgrade its 3G EV-DO network from revision 0 to revision A to add upstream capacity and lower latency in the broadband network. The changes would take affect in 18 Alltel markets and will bring the network in line with Verizon.

Alltel also joined its wireless competitors in offering unlimited voice plans back in March as an alternative to subscribing to a MyCircle plan. Called Freedom Unlimited, the $99 per month plan serves as an alternative to its MyCircle options. Today’s announcement puts the focus back on the Circle. In addition to unlimited texting, Alltel is also encouraging usage of other wireless data services such as its widgets to supplement revenue from SMS, something that could ultimately help Verizon upon acquisition.

Based on today’s announcement, those in My Circle programs of five, 10 or 20 members can choose from plans for $7.99 or $12.99 for 400 or 1,000 additional text, video and picture messages, respectively, to message those outside of their circle or on other carrier’s networks. This will be offered as an alternative to Alltel’s All Access Pass, available for $19.99 for unlimited messaging and mobile Web.

To accommodate families as well as friends, Alltel is also giving Family Choice Plus plan customers unlimited mobile-to-mobile calls and unlimited nights and weekends. The plans run at $84.99 per month for two lines with 1,000 shared minutes and the ability to call any 10 numbers on any network. Customers can also receive two lines with 2,000 shared minutes and unlimited calling within their circle of 20 numbers for $114.99. The plans now come with the choice between free evening calls starting at 7 p.m. or 1,000 text, picture or video messages.