I think the comments made earlier regarding a few drops of rain resulting in an arrival Hold at Atlanta a little inaccurate. I am not a controller obviously, and have limited experiance in the matter, but from what I have observed on that website, Even when the weather is marginal, approaches are made with relative safety.
Giles asks a relavent question, Does an aircrew declare a full emergancy when dealing with a medical situation on-board? A full Mayday or Pan Pan?