I strongly doubt any 'tieing' between Xbox shipments and software: mostly just because I don't think Microsoft is that integrated (i.e. the console business and sales team is totally different from the PC software side)... not to mention the idea of any retailer not getting exactly as much software as they order just doesn't happen (i.e. there's *always* enough Vista, Office, etc. to go around when all you have to do is press more discs).
Rather I suspect MS is doing deals with retailers: look at all the ad hoc 360 sales that happen at various retailers all the time, that MS has to be paying for in the end. These pack-in deals are also worked out with retailers.