I came over this issue, while extending a webservice written in .net 1.1 with a 2.0 - plugin.
I implemented the correct interface in my plugin, which is needed to be recognized by the plugin loader. Once the plugin was deployed I saw, that the plugin was not loaded at app-startup.
I wanted to investigate, why there was this behavior. So I connected to the iis process and wanted to debug the 1.1 app and the plugin loader method. The problem was, that it is not possible to debug a 1.1 webservice app with Visual Studio 2005. Connecting with VS 2003 works, so I found out, that reflection didn't recognize my plugin (the implemented interface).
I thought that there was something called side-by-side execution for 2.0 and 1.1 assemblies, but it's not working by default for web-apps.
The solution is to go to the inetmgr mmc and switch the ASP.Net version from 1.1 to 2.0 in the ASP.NET tab.
After manually selecting the framework version the 1.1 webservice app was able to recognize my 2.0 - plugin via reflection.