Windows

Boy.. this is a huge decision, but I would recommend a single master domain setup. In other words, create one AD tree (like MyCompany.com). Implement AD on one server at each site with replication setup between the 2 remote sites and the parent site. Schedule replication traffic to occur after hours to reduce traffic during peak hours. Install Exchange Server in the parent domain. Create complete (2 way) trust relationships between all sites. I would store all users in the parent location AD server and replicate the database to the remote servers. This will create more traffic during logon, but will provide easier management. It shouldn't be a big deal with full T1's at each site. Email me at chrisp@metronational.com if you have anyother questions. Good Luck.

This questions eerily familiar...
Search for my question "Time to upgrade, where is my hammer?" a few people had some good links and answers there.

http://www.techrepublic.com/trbbs/message_detail.jhtml?thread_id=10449&thread_title=Time+to+upgrade%2C+where+is+my+hammer%3F&ooc=open

re site links

connections can not be made between computers in sites unless there is a site link in place. have each site have a link with other 2 sites A has site link B and C.
B has site link to A and C. ..

the replication protocol can berpc or smtp.
rpc replication can be scheduled. smtp is asynchronous and ignores schedules. smtp requires you to install and configure a certificate authority (CA). The CA signs SMTP messages exchanged between domain controllers. This insures authenic directory updates (really important since you are linking via internet rather than private wan). rpc does not require a CA.

-Wayne