<p>I don't use Eclipse as an IDE, and have no interest in doing so.  However, I do like its source-level debugging.</p>

<p>Is there any way I can use it to debug a C++ Linux app without going through the ritual of creating a project?  (In effect, can I just use it like a frontend to gdb?)</p>

<p>If not, what are the steps I need to follow to create a project that I can use to just debug an existing C++ program that is built using Makefiles or other tools (SCons, CMake, etc.).  I don't want to be able to "develop" in Eclipse; all I need to do is debug.</p>

Take a look at
this question
. Create a C/C++-project, use your project's source directory as project directory, select to use the external builder, and change "make" to whatever tool you want.
The tricky part is to get the indexer to work correctly and find all your header files.
EDIT: CMake 2.6.x has support for generating CDT project files, which might be a more straightforward solution.