So I finally got some time to pursue this. I SVN updated my project and did a clean/build and it still wouldn't run the debugger. My intention was to check out a fresh copy of the same project, build and test it and start stripping things out until it worked.
Seemed logical, except it worked the first time I built it . I did a recursive differences on the two working directories, but I'm unable to tell what the root cause is. They do both generate different .hex files, but it's not obvious to me why, as the source files appear identical. I'm going to delete my first working copy, declare victory and move on.
Thank you to everyone who tried to help.