Different revisions of the .NET framework can lead to incompatibility problems with your application. You might have used a feature of the 1.0 framework which was later changed in the 1.1 release, or perhaps used a new feature in 1.1, not available earlier.
ASP.NET Version Switcher is a really handy tool for running web applications against different versions of the framework. Use the tool to be confident your users will be able to deploy your apps, no matter what their .NET framework version. Excellent stuff!