Motorola has reconciled its differences with the Symbian operating system, taking a 50% stake in UIQ Technology, the Sony Ericsson-owned developer that builds user interfaces on Symbian software. The two companies will form a venture to jointly run and invest in UIQ’s middleware and open interface platform, Motorola and Sony Ericsson said today.

The move could signal a huge shift in the smartphone market in the U.S., which unlike in the rest of the world has been fragmented among multiple operating systems, including the Palm OS, Microsoft’s Windows Mobile and Research in Motion’s BlackBerry platform. While Symbian is the worldwide leader in smartphone platforms, the U.S.’s lack of exposure to Nokia—Symbian’s biggest proponent—and to a lesser extent Sony Ericsson, have kept the Symbian OS a relatively minor player domestically. Motorola, however, is the largest manufacturer of phones in the U.S., and its investment in UIQ may result in a flood of new Symbian devices in its home market.

Motorola has used UIQ in one of its phones before, the Z8 Multimedia phone released in Europe earlier this year, but so far Motorola’s smartphone strategy has been a mishmash of operating systems and faltering alliances. Motorola was one of the founding members of the Symbian joint venture, along with industry heavy-hitters like Nokia, Matsushita and Ericsson, but it dropped out in 2003 to nominally pursue a smartphone strategy based on Windows Mobile, the open-source Linux platform with Java-based user interfaces and applications. That resulted in Motorola creating numerous high-end feature phones with closed operating systems such as its enormously popular RAZR, but few true smartphones—a market that came to be dominated by Nokia along with specialty device makers like RIM, Palm and HTC. The smartphones it released, powered by Linux, became popular in Asia but never made it across the Pacific. And its U.S.-targeted smartphones, including the Q, incorporated the Windows Mobile platform.

Sony Ericsson itself bought UIQ from Symbian only last year, setting up a rival interface to Nokia’s Series 60 UI. Sony Ericsson said at the time it wanted to set up a UIQ community with open licensing to all vendors as well as additional UIQ investors. Motorola is the first to take Sony Ericsson up on its offer, not only licensing the software for the Z8 but buying into the company.