Rumors of a new forthcoming iPhone could provide AT&T with a positive jolt, given the historic benefits the carrier has enjoyed from Apple’s multi-purpose mobile device.

For investors, this could be a good time to get in early on the potential windfall. Currently trading at slightly more than $24.00, AT&T (NYQ: T) is hovering near the bottom of its 52-week range of $20.90 – $40.70.

If Apple does in fact introduce a new iPhone, it could be faster and cheaper than current offerings, or create a new low-priced niche that matches the performance of its current 3G model.

Regardless, the impact could only be good for AT&T, which is the exclusive U.S. network provider for the iPhone.

The latest buzz about a new iPhone originated from United Arab Emirates mobile carrier Etisalat. When it announced on February 4, 2009 that it will begin selling the iPhone 3G on February 15. A report on the contract also mentioned a new iPhone slated for June.

The source of the June iPhone release remained murky, but analysts have been saying that a new iPhone may hit the streets this summer. The investor play here would be a long bet that the rumors are in fact true, and that history would repeat itself when it comes to Apple’s iPhone being the best friend of AT&T shareholders.

Going back to January 2007, when Steve Jobs introduced the iPhone to the world, AT&T saw a tsunami of subscriptions.

AT&T had said it that it signed up 146,000 new subscribers in the first two days it offered the iPhone. About 40% of subscribers who purchased an iPhone were new AT&T customers, according to the company. AT&T executives also said at the time that “store traffic above historical levels.”

The profitable relationship continues…

In its Q4 and full-year 2008 earnings, AT&T reported that quarterly net income was down 23.3% year-on-year, with $2.4 billion in 2008 compared with $3.1 billion in 2007.

Wired voice-service revenue dropped from $9.8 billion in Q4 2007 to $8.8 billion in Q4 2008 – a decline 10.3%.

However, full-year net income was up. It increased 7.7%, from $11.95 billion in 2007 to $12.87 billion in 2008.

The company said the increase was largely attributable to wireless revenue, which shot up 13.5%, from $10.2 billion in Q4 2007 to $11.5 billion in Q4 2008. AT&T reported one million additional subscribers in Q4, resulting in a total of 77 million – an increase of 7 million in 2008.

The growth was directly related to iPhone sales. AT&T activated 4.3 million iPhones in the second half of 2008.

Apple’s iPhone played a major role in the 51.2% growth in wireless-data revenues in Q4. According to AT&T’s press release, “wireless integrated devices in service more than doubled over the past year.”

While analysts argued over whether or not those numbers met expectations, the bigger theme here is that a new iPhone could increase AT&T’s revenues in 2009. One way or another, that has to give the stock a jolt barring any unforeseen debacles in these crazy times.