Future Sony Ericsson phones launched under its new Xperia brand may not be based on Microsoft Windows, the handset maker said at the launch of its digital marketing campaign for the Xperia X1.

The X1, which will be the premium phone in Sony Ericsson’s portfolio when it starts shipping later this month, is Sony Ericsson’s first smartphone powered by Windows Mobile and was seen as boosting Microsoft’s presence in the smartphone market.

But the loss-making joint venture of Japan’s Sony and Sweden’s Ericsson was reluctant to reveal any details of future models that may be launched under the Xperia brand, of which the X1 is the first.

“The brand is not tied into any specific technical platform,” said Magnus Andersson, product manager for the X1.

Asked how much pressure Xperia was under, given Sony Ericsson’s shrinking range of must-have products, Andersson said: “I think there is a lot of hope riding on everything Sony Ericsson does.”

Geek experts commenting on the model and its future in the U.S. market – not likely to debut before November – felt that AT&T’s networks best-suited the X1. Uh, O.K..