Ok some progress, a really awesome user swapped out the release library with the debug version and sent me the Allegro log files and a dump file I could open fully.
Here's the full stack trace:
I'm using a multimonitor setup on at two test machines including my dev machine, plus at least two testers used multiple monitors without seeing the issue. That said at least four people on my forum are experiencing the issue, I'm waiting to hear back as to what hardware they all have but I'd guess it would be fairly different. We will see!
Trent: Send me a mail to firstname.lastname@example.org and I'll send you a build key to test
I don't know if Allegro was built with this flag I'm afraid, I'm just using the official version from http://cdn.allegro.cc/file/library/allegro/5.0.10/allegro-5.0.10-msvc-10.0.7z .
Logs and dump file : http://www.ionisingsoftware.co.uk/isomer_worklog/isomer_0803_dump_fullscreenissue.zip