This is because Sony-Ericcson has an established common firmware that is common with all sony-ericcson, nokia, and other mobile phones. Because of a common platform that is used among loads of phones, there is a wide community that has this platform, so development is very likely.
Anyone with ITRON OS? :D :P
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Lots of info for SE P series ie P800, P900, P910 and Linux here:
http://www.alfonsomartone.itb.it/pdzrho.html
:)