While it's true that the .NET 3.0 doesn't directly add any ASP.NET functionality, it's also true that the .NET 3.0 install makes changes in your \Framework\v2.x directory.  Although the install introduction warned that a reboot might be necessary, it didn't turn out to be necessary and the install was relatively painless.  ...Untill we tried to install on a server running a 2.x webservice and received an error indicating that assembly ServiceModel.DLL couldn't be found. 

The primary assembly for Windows Communication Foundation (WCF) services is ServiceModel.DLL. Another seemingly unrelated fact is that hosting a WCF service in IIS requires a service file with a ".svc" extension in a virtual directory.  How is the svc file processed?  An Http handler for the "*.svc" extension is associated with ServiceModel.DLL.  And how does the .NET 2.0 based ASP.NET know to suddenly start looking for this service file?  The 3.0 install adds a new http handler to the web.config file in the \WINDOWS\Microsoft.NET\Framework\v2.0.50727\CONFIG directory.

<configuration>
  <system.web>
  ...
    <httpHandlers>
    ...
 <add
           path="*.svc"
           verb="*"
           type="System.ServiceModel.Activation.HttpHandler,
           System.ServiceModel, Version=3.0.0.0, ...

An IIS restart allowed the web service to resume without error.