Problem:

Will an application built for the the .Net Framework V2 using Visual Studio 2005 run successfully on .Net 3.0.

Resolution:

This will work unchanged.

Microsoft have not changed the Common Langauge Runtime (CLR) from .Net 2.0. It is still v2.0.50727.

The .Net 3.0 functionality has been as libraries that exist on top of the CLR V2. As such your application will run correctly on machine with .Net V3 installed.