A follow-up to this older post, for future reference:
I experienced the same problem with a project I'm working on -
incremental linking would never kick in despite having the project
setting turned on, even when I made a trivial change to a non-library
cpp file.  This is using VC++ 7.1.

In the project I was working on, I had the linker set up to ignore the
default library nafxcwd.lib, and then I explicitly added it to the
libraries to link against list.  This was to prevent some "new" and
"delete" symbol conflicts in MFC's library and the C runtime library
(see Microsoft's Knowledge Base article Q148652 for more info).

I discovered that this caused the linker to think that project settings
had changed every single time it went to link.  By changing the linker
settings to not ignore the default library nafxcwd.lib, the project
still linked successfully, and this time incrementally, as well.  Note
that this means I was not exactly following Microsoft's recommended
practice in their KB article, but it still links and runs okay -
perhaps that KB article is geared toward an older version of their
linker.

Matt