Gah, bloody typical.  It’s Friday evening and I’ve just discovered a game breaking bug.  In both the standalone and web-player builds, it seems Unity sometimes decides not to call my “on collision stay” callback, which breaks the whole construction mechanic – it means you can’t attach parts together!  I’m not sure how long it’s been like this, I usually test the game in the editor, which naturally enough doesn’t have this issue.  Lesson learned: I really should properly test the standalone builds more often!  Hrm, it could be a while before I figure this one out… 🙁