Plenty fast, according to promises made by AT&T (T) mobility chief Ralph de la Vega at Morgan Stanley’s annual Communications Conference on Wednesday.

De la Vega said that a version of the network was already running in AT&T labs at 7.2 megabits per second, which according to AppleInsider‘s Katie Marsal is double the theoretical throughput of the company’s HSPA (High Speed Packet Access) network. (link)

It also happens to match the specs of the Infineon (IFX) SGOLD3 chip Apple has reportedly chosen to serve as the new iPhone’s cellular modem. See here.

Throughput of 7.2 Mbps would put the 3G iPhone within spitting distance of Wi-Fi speeds, which typically run between 6.5 Mbps and 20 Mbps.

But de la Vega didn’t stop there. According to Marsal, he told the Morgan Stanley audience that sometime in 2009 the company will transition to HSPA release 7, which could deliver speeds “exceeding 20 megabits per second.” (link)

Of course, we won’t know how fast Apple’s (AAPL) 3G iPhone really is until someone gets their hands on one and runs some good benchmark tests in the wild. AT&T EDGE network, after all, is rated at up to 236.8 kbps, but when put to the test, actual throughput turned out to be in the 50 to 90 kbps range (see here).

In February, AT&T said it expected to deliver 3G services to some 350 leading U.S. markets before the end of 2008, including all of the top 100 U.S. cities (link). Owners of 3G iPhone in the other U.S. markets will have to make do with EDGE — or whatever stray Wi-Fi signals they manage to pick up.