Shared calendar across domains

I have a client who is going through a merger with 2 other companies.  Each company has it's own Exchange server and email domain.  The companies would like to have shared calendars between domains.  Essentially,  would like to share a calendar with and  

I know how to do this within a single domain, but not across domains.

Would it be best to take these email domains to the cloud with hosted Exchange or use an existing server at one of the companies to host email for all domains?  How would one create such a setup on an onsite server?

Who is Participating?
If you can install, or already have Exchange Server 2010 with Client Access server role, you can then use the Federation Sharing, that allows to share Calendar and Contacts between external Exchange Domains.
perk83Author Commented:
So in that scenario, would each site need an exchange 2010 server or would we need one exchange 2010 server that would host all email domains at one location?

Two of  the companies already have exchange 2010 server and the other is still on exchange 2003.  The exchange 2003 company has the majority of the users, about 60 that we would need licenses for.  At $85 per license, that would be $5100, not counting any tech time for the setup or moving of databases.

Wouldn't it be better to take all three to the cloud (hosted exchange) for $4 a month per user?  120 users would be $480 a month.  Can't you have multiple domains in the same account on hosted exchange?

Thanks so much for your knowledge and input.  I really appreciate it.
All Domains needs to have Exchange 2010.

As for the Exchange 2010, i think user licenses are around 50 dolares, would make $3000.
I cant answer about the multiple domains in Exchange Cloud but makes sense that would be allowed.
But in Cloud, after 6 months, you would have spent the same $3000...
Its not a easy decision...:)
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perk83Author Commented:
So for the one company that has Exchange 2003, we would need Exchange 2010, along with the correct number of licenses?  That would most likely mean additional hardware since our current EX2003 server is about 5 years old.
Yes, only the Exchange 2003 users will need new 2010 licenses.
Probably need new hardware, because 2010 requires x64 CPU...
perk83Author Commented:
Thank you for the info.  I really appreciate your guidance and input.
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