Shipments of Wi-Fi-enabled phones continue to double every two years, according to a report from ABI Research.

The wireless industry is on track to see 144 million dual-mode cellular/Wi-Fi handsets shipped this year, with forecasts for 2011 at just over 300 million. That growth continues a trend the research firm identified in February, when it found that dual-mode cellular/Wi-Fi handset shipments were set to double between 2008 and 2010.

ABI industry analyst Michael Morgan attributes the growth in part to changing attitudes on the part of operators. Carriers once feared that Wi-Fi would take traffic off their networks, resulting in lost revenue. As data demands have increased, that attitude has changed, and Wi-Fi is now being viewed as a way to free up available network capacity.

How operators view Wi-Fi is largely a function of their particular circumstances, Morgan says. “Verizon has not enthusiastically embraced Wi-Fi in its handsets, while AT&T has,” Morgan says.

The heavy data usage of AT&T’s iPhone subscribers resulted in a dramatic increase of Wi-Fi. Verizon hasn’t faced that type of situation yet, but Morgan says it is warming up to Wi-Fi.

Morgan also says that Wi-Fi’s benefits depend on a carrier’s circumstances. T-Mobile USA, which has no landline assets, uses Wi-Fi (via Hotspot at Home access points) to deliver an improved in-home service that it couldn’t achieve before. In contrast, AT&T does have landline assets, so Wi-Fi’s benefit is to take a load off AT&T’s cellular network.

“The picture may be unique to each carrier, but in the end, Wi-Fi can offer most operators those two key benefits: extended reach and/or network load reduction,” Morgan says.