I installed the Gallio bundle Version 3.2 Build 517 in order to use MBUnit. My tests are running fine when I try run them through the Icarus test runner, but when I hit the debug button on the test runner, it tells me it could not attach to the host. If I add some break points and attach the Icarus process to Visual Studio, the break points do not hit and it tells me that no symbols are loaded.
It's possible to have this problem if your test dll that references Galio / MbUnit dlls in different directory than where Icarus is installed and the dlls have different version.
If it's the case just make sure that versions are the same in both folders.