Debugging managed COM objects being called from VBA

I've found I can open my test spreadsheet automatically when i start
my VS project using the debug properties page of the project. I've
found a new showstopper though. The moment I try and run the calling
VBA macro in my spreadsheet it closes excel and ends my VS debugging
session. The same thing happens if I have it launch excel without a
document and then manual open it using that copy of excel.

If I open the spreadsheet in a separate excel window the macro runs
and calls the COM object successfully but my breakpoints in the C#
code are ignored.

this is happening with VS2003 and both Excel03 and 07.