The logical architecture of Sharepoint 2010 has changed significantly from MOSS 2007.  From the session I attended yesterday at SPC09 the key improvements were highlighted as follows:

• Flexible deployment model
  • Pick which services in which environment
• Improved security model
  • Claims based authorisation
  • Cross-farm communication via web services
• Simplified administration model
  • Managed via central admin and PowerShell
• Service isolation
  • Each service app uses separate database and optionally different app poll
  • Support for multiple service applications for a service with different accounts and databases
• Multi-Tenancy
  • Some service apps can be partitioned to handle multiple tenants

Also mentioned in the session was the fact that Microsoft have released a set of technical diagrams and models relating to planning logical and physical architectures in SharePoint 2010 and upgrading from MOSS 2007.