I get asked this quite a bit for a reference architecture.  We just published our MS IT Whitepaper on how and what we did for deploying 180,000 mailboxes on Exchange 2010.  There is excellent information in there around Exchange 2010 best practices, architecture considerations, DAG and storage design, backupless strategies, UM design, user training, etc.  I snipped a few shots from the whitepaper:

 

Our sample hardware we leveraged:

Microsoft IT used the following hardware per server role:

· Client Access Two quad-core Intel Xeon x5470, 3.3 gigahertz (GHz), with 16 GB of memory.

· Hub Transport Two quad-core Intel Xeon x5470, 3.3 GHz, with 16 GB of memory.

· Mailbox   Two quad-core Intel Xeon x5470, 3.3 GHz, with 32 GB of memory.

· Unified Messaging Two quad-core Intel Xeon x5470, 3.3 GHz, with 16 GB of memory.

 

Our hub routing design:

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JBOD Storage architecture we deployed for Exchange 2010 using anywhere from 4 to 16 node DAGs:

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Our DAG design showing ~3,000 mailboxes per DAG node:

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Our UM design showing how UM can scale. We leverage only 5 UM servers for 85,000 endpoints:

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How we designed highly available UM:

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Grab the whitepaper here.