Microsoft has released two new "tested solutions" whitepapers for Exchange 2010. If you've not seen these before, they are whitepapers that, according to Microsoft, "provide examples of well-designed and cost-effective Exchange 2010 solutions deployed on hardware offered by some of our server, storage, and network partners".
Previously, Microsoft released papers for 500, 9000 and 16000 mailbox solutions on hardware such as Dell servers, F5 load balancers, etc. There are now papers for 15000 and 20000 mailbox solutions:
Exchange 2010 Tested Solutions: 15000 Mailboxes in Two Sites Running Hyper-V on Unisys ES7000 Servers and Hitachi Adaptable Modular Storage 2000 Family
Exchange 2010 Tested Solutions: 20000 Mailboxes in Two Sites Running Hyper-V on Dell R910 Servers, EMC CLARiiON Storage, and Brocade Network Solutions