Exchange Calendar Sharing between unrelated Domains

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http://serverfault.com – I have a 'request' from one of the 'big guys' that has me scratching my head a bit. He is one of our Executives that also is a board member of several other organistations, so he floats around between 4 different unrelated companies, each with their own domains, Exchange setup etc. He has domain accounts in each organisation. He has an iPad with multiple Exchange accounts so he can see all his calendars which works Ok for him - Apple calendaring bugs/flaws aside. What he wants is the ability for 'reception staff' at each organisation to 'see' all his calendars as they are booking things for h (HowTos)