It will never work out due to money. Sony, Microsoft, and Nintendo will always make it difficult on developers to even fonder the idea of making a cross-platform game. The only thing I see working would be a game that is playable over Xbox and Games for Windows (like Shadowrun).
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Yes, that certainly is an issue and it's a damn shame it is too, but even Microsoft aren't utilising the cross-platform system properly. GTA IV is a Windows Live game on the PC, so why go through the fuss of making it use the Xbox guide system when it can't actually join Xbox players?