In 2006 Microsoft announces that the new user interface (to become the RTC) will not be part of Navision 5.0, as previous announced. Instead Morten Elsberg announced that it will be released 6-12 months later as Microsoft Dynamics NAV 5.1.
Version 5.1 would be the first product that would offer the new 3-tier environment. With a new client as the presentation/gui layer handling user interactions. The actual business logic is handled on the IIS layer. The last tier, the database, is handled by SQL Server 2005.
Microsoft never releases a version 5.1 (only a 5.0 SP1). Instead the version with the new user interface is released as Microsoft Dynamics NAV 2009.