For some reasons I cannot access to Steam overlay and other Steam features (F12->screenshot) while playing Dungeons of Dredmor. Do you know how to fix it?
Just to be precise the in-game option in the Steam game properties is enabled.
As written here by developers is a known bug.
Update: It will be fixed in next patch.
When the patch 1.0.3 will be live add
In the first post on this page says specifically that it crashes in fullscreen. Have you tried pressing shift-tab when you were not in fullscreen?
This would only be a temporary solution, but it has been reported here on the developer's support site as a bug, so it may be fixed in the future.