Meego will no longer be continued by Nokia. Elop wanted to create and "ecosystem" of the Windows OS in Nokia phones and PCs. Meego will be a trouble if they support it as it will compete with their key products.
The Windows 7 OS will be for high end phones. Nokia does not plan to be like Apple who only sells high end smartphones. By the time Sony Ericson becomes Sony (Ericson will or is already selling it's remaining shares of stock to Sony), the said company plans to sell smartphones only. Was that not interesting?
Symbian will be for midrange and lower end phones. Symbian will be slowly discontinued and will be forgotten by 2016. For now, Nokia has promised us with lots of updates. My N8 originally came with Symbian^3, I updated it to Symbian Anna, and Nokia will release the Symbian Belle update for it on the first quarter of 2012. For Symbian phones with 1GHz processors, Nokia will provide them with the Symbian Carla update, and future Symbian phones with dual-core processors will have the Symbian Donna update.