It is a possibility according to an interview with Pierre Perron, Sony Ericsson France General Manager:

“Mobifrance: Xperia X1 loads Windows Mobile. Are all models of this range will work with this operating system?

Pierre Perron: Not only, we work with all operating systems. We even share important in the UIQ platform. However, with Xperia, we send users “Experiencers.” This clientele demanding that wishes to all the technology and innovations in their mobile terminal without compromise. The range will be enriched Xperia
Other smartphones running Windows Mobile or Symbian or UIQ or other which will complement this range. “

The French translation isn't the best, here's the original for those who understand, maybe later on M. Hussein can translate:

“Mobifrance : Xperia X1 embarque Windows Mobile. Est-ce que tous les modèles de cette gamme fonctionneront avec ce système d’exploitation ?

Pierre Perron : Pas uniquement, nous travaillons avec tous les systèmes d’exploitation. Nous avons même des parts importantes dans la plateforme UIQ. Toutefois, avec Xperia, nous adresserons les utilisateurs « Experiencers ». Cette clientèle exigeante qui souhaite toute la technologie et les innovations dans
leur terminal mobile sans compromis. La gamme Xperia sera donc enrichie d’autres smartphones sous Windows Mobile ou Symbian ou UIQ ou autres qui viendront compléter cette gamme.”

Well there is nothing wrong with Symbian UIQ, although this line could have been an exclusive of Windows Mobile. Nothing is for sure, and Perron didn't confirm anything either, but the possibility stays here.

Picture: LetsGoMobile

Source: SymbianFrance via The UIQ Evangelist