Chris,
Great plan - having already done a few R2 PBX replacements I know you'll be happy with the solution. Here are the answers to your questions:
Will we be able to register our Polycom phones with OCS?
Not presently. My company is nearing completion of a product that will, among other things, allow you to use standard SIP phones with OCS. Stay tuned.
What would we need to do to them to make them work?
Is there a list of compatible devices anywhere?
http://technet.microsoft.com/en-us/office/ocs/bb970310.aspx
What issues can we expect with the phones being on a different network to our user's PCs? Would it just be a case of situating the OCS server on both networks?
Our solution would require the Mediation server to be on both networks.
Are there any special requirements for the telephony provider to support SIP trunking from OCS as opposed to other providers, or is it a standard method that they will support "out of the box" as it were.
As of the R2 launch there are only 4 SIP providers that are compatible with R2 "out of the box." OCS requires the SIP traffic to take place over TCP, which is not widely supported today. Our solution will allow you to connect with any (or multiple) SIP trunk provider(s).
We will be actively seeking beta participants very soon in a widespread fashion but in the meantime your organization seems to be a perfect fit for the beta. If you are interested, please send a message to smartsip at evangelyze dot net.
Mike Stacy | Evangelyze Communications | http://www.evangelyze.net/cs/blogs/mike