Mark,
It looks like the ceiling in the Metrodome is treated like "air" - that is, a batted ball that hits the ceiling is treated as "in flight". It's not an "automatic" home run, but it could become one if it continued out of play in fair territory after hitting the ceiling.
You can find the ground rules for the Metrodome and all of the other MLB parks at the link below.
http://mlb.mlb.com/NASApp/mlb/mlb/of...ound_rules.jsp
In terms of local fields with overhangs extending over fair territory, every time I've played on one a batted ball hitting the overhang has been treated as foul by ground rule. If I were a visiting manager and the home manager suggested this be treated as fair, I would object per OBR 3.13.
JM