System.Web Hi-Jacked

I tried adding a trace listener to a web application to determine what's causing the heavy view state on the page. When I added the WebPageTraceListener using the type attribute
type="System.Web.WebPageTraceListener, System.Web"
the application simply stopped as it couldn't find WebPageTraceListener in the System.Web assembly. I used the Object Browser and the class was right there.

I ended up pasting in the type from another project that I had, which included the version and public key token like so:
type="System.Web.WebPageTraceListener, System.Web, Version=2.0.3600.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"
and that fixed the issue. I'm guessing there's a different System.Web assembly being used (I created a blank VS2008 solution and simply added the VS2005 projects, so that could be it).