During the Bank of America Merrill Lynch Media, Entertainment & Communications conference, AT&T CFO John Stephens said that the company plans to officially launch its LTE 4G network on September 18 (Sunday). As previously stated back in May, the launch will initially take place in five major cities across the U.S. including Atlanta, Chicago, Dallas, Houston and San Antonio.

Currently only three AT&T devices are compatible with the upcoming network: the $699.99 HTC Jetstream tablet, the $69.99 Mobile Hotspot Elevate 4G and the $49.99 USBConnect Momentum 4G modem. AT&T has also stated that its HSPA+ USB modem, the USBConnect Adrenaline from LG, became upgradeable to LTE starting August 26 via a software update.

The network’s first LTE smartphone is expected to be the HTC Holiday, arriving within the next few months. It will feature a 4.5-inch qHD (540 x 960) display, a 1.2 GHz dual-core Snapdragon SoC, an 8MP camera and Android 2.3.4 (overlaid with HTC’s Sense 3.0 interface). If you assume that the next iPhone also has LTE technology then they may very well be the first LTE phone in AT&T’s lineup, if they are launched in October as we assume.

For now, the new network won’t come cheap to subscribers: access will cost a meaty $50 for 5 GB of data. Rollout will also be somewhat slower than Verizon’s approach, with an estimated 70 million potential users in 15 cities by the end of 2011.

Verizon’s network actually launched during December 2010 in 38 cities simultaneously, and seemingly adds a new batch of cities every week – 26 new areas gained access to Verizon’s 4G network on Thursday alone. Verizon claims that its LTE network now covers more than 160 million access points and is available in 143 markets. Verizon expects to engulf 175 markets and 185 million access points by the end of the year.

So how fast will AT&T’s new 4G network go? Verizon says that its own LTE 4G network supports average data rates of 5 Mbps to 12 Mbps for downloads, and 2 Mbps to 5 Mbps for uploads. AT&T hasn’t revealed any actual numbers, but there are reports that its connection speeds will be similar to its rival’s 4G LTE network. AT&T merely states that “data rates can vary” instead of offering actual numbers.