Google’s Android mobile operating system gained 14 new converts today, most notably handset maker Sony Ericsson, adding further momentum to Open Handset Alliance’s efforts to reshape mobile computing. Though Sony Ericsson did not say if the decision would affect its commitment to other smartphone platforms such as Symbian, the vendor confirmed its intention to design at least one phone with Android OS.

Sony Ericsson, which became the world’s third-largest handset vendor last quarter, joins fellow handset giants Motorola, Samsung, LG Electronics and HTC, the maker of the first Android-powered handset, the T-Mobile G1. Chinese handset vendor Huawei Technologies as well as embedded electronics companies Garmin, Toshiba and ASUSTek and mobile operators Vodafone and Softbank Mobile also announced their membership, further bolstering Android’s ranks. But Sony Ericsson’s supportwas particularly key, as it was the last major handset vendor outside of Nokia to get behind the open-source, Linux-powered platform.

Nokia, however, is still a big exception. It not only dominates global handset sales with 38% market share, it is also the biggest manufacturer of smartphones, through its Nseries and Eseries lines. Nokia has been the biggest proponent of the Symbian mobile operating system, which powers all of its smartphones exclusively. But In response to new openly licensed and royalty-free platforms like Android and the LiMo Foundation’s Linux Mobile, Nokia decided to buy up the remaining shares Symbian it didn’t already own and then spin the company off as a foundation distributing the software for free. Nokia completed the acquisition last week and plans to offer up its first open Symbian license in the second quarter.

How an open Symbian will compete against other open-source platforms largely depends on the decisions of Symbian’s other vendors. Sony Ericsson was half-owner of UIQ, a middleware company that designed the user interface for Sony Ericsson and Motorola’s Symbian phones. With the creation of the Symbian Foundation, however, UIQ is being jettisoned in favor of the Nokia-developed S60 interface, leaving Sony Ericsson win no direct stake in Symbian. When UIQ was shown the door, Motorola took the opportunity to abandon Symbian entirely, announcing in October it would build its future smartphones with the Android and Windows Mobile operating systems.

Sony Ericsson joining the Open Handset Alliance could mean a dramatic shift toward Android or other operating systems, but it could have a much more muted effect on its future portfolio. Many handset vendors like Samsung and LG support numerous operating systems, which they sell to different carriers and different markets. Sony Ericsson could merely be including Android among its numerous supported operating systems. The vendor, however, has shown signs of distancing itself from Symbian. In September, Sony Ericsson reorganized its software development, scaling back on UIQ-Symbian development while adding staff to its Windows Mobile Group in California. Furthermore Windows powers Sony Ericsson’s most anticipated new smartphone, the Xperia. Conversely, Sony Ericsson has announced no plans to build any new Symbian phone models using the new open-source S60 software.