I've heard in rumors that PSN and XBL users will be able to play cross-console/cross-company with each other. I do not know if this is confirmed or true but if someone does know, please let me know (preferably a Bungie employee).
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Its not possile with Destiny but I really wish companies would do it more often. Final Fantasy XI was cross platform between PC, PS2 and Xbox 360. But thats because PlayOnline completely webt around Xbox Live, and PSN wasnt a thing yet back on PS2.