There are a couple of places where the GCC "visibility" isn't implemented correctly/completely.. This shows it's self as missing symbols when trying to link Python-Ogre against OgreMain..
There are 2 cases of classes missing _OgreExport
And as Singleton's are implemented as templates it also needs to specifically be set a visibility(default) -- it seems by default gcc makes templates 'hidden'
This has been tested with GCC 4.1.2 under Ubuntu - and with VC7.1 on windows where it has no effect :)