Issue 1: Remote Debug
You can't remotely debug an application from Visual Studio 2008 when the installed debugger on the remote (target) machine is from the Visual Studio 2005 install. You must install the Visual Studio 2008 remote debugger on the target computer to attach to the remote debugger from the Visual Studio 2008 IDE. I believe side-by-side installation of the VS 2005 and VS 2008 remote debugger is fine. However, I don't believe they can run concurrently.
Issue 2: MSTest
If you try to run the VS2005 version of MSTest against a test assembly compiled with VS2008, MSTest will report that there are no tests to run. To resolve this issue, install VS2008 components on the remote computer and then from a VS2008 command prompt, run MSTest.
These issues drove me crazy for a while, so hopefully it will help you avoid an assault on your patience.
Technical Speaking author, Ethan Wilansky