SAN FRANCISCO – CEOs from Sprint, T-Mobile and Verizon Wireless extolled the virtues of open networks –  as long as those networks don’t get too open – Wednesday at the CTIA wireless conference in San Francisco.

The executives took the stage to discuss “openness” –  letting consumers use any mobile device or application over any cellular network. Wireless carriers have traditionally served as gatekeepers to their networks, deciding which applications and even which websites their subscribers  can access.

But that’s all changing. Well, sort of.

T-Mobile CEO Robert Dotson  told the audience that while the so-called “walled garden” is a thing of the past, carriers need to retain some control to ensure “security and privacy” and provide a reliable experience for consumers.

“There needs to be some stewardship and control,” said Dotson, who argued that a mobile operator can’t guarantee services like voicemail and multimedia messaging will work smoothly when consumers use a phone that’s not optimized for their network.

Sprint (S) CEO Dan Hesse said that in March his company began allowing full Internet browsing, meaning that Sprint subscribers can now look up any website on their mobile browser, even those that aren’t “optimized” for mobile use.

Verizon (VZ) CEO Lowell McAdam also plugged his company’s commitment to openness by pulling out of his pocket two non-Verizon devices (a $69 phone from prepaid service provider AirVoice and a wireless router for the insurance industry) that are currently running on the company’s network. Verizon recently opened its network to outside handsets that meet its minimum testing standards.

Despite vowing their allegiance to openness, all three CEOs echoed claims that truly opening up their networks wouldn’t be all that beneficial to consumers.

McAdam, for example, argued that separating phones from service plans would mean people will have to pay more for cell phones that are now heavily subsidized by carriers in exchange for two-year service contracts.

What’s more, said McAdam, people are accustomed to being able to walk into their carrier’s stores when they have problems with their device. With a more open system, the carrier wouldn’t be involved with the sale of all devices running on their networks, and thus wouldn’t be too keen on helping customers resolve hardware or software issues created by other companies.

“When an application crashes on a Dell laptop you don’t call your cable provider,” said McAdam.