It seems reasonable that if dot-notation works as a field-selector when selecting object members, that it should also work when selecting array indexes,
but that doesn't seem to be the case with the current version of MongoDB.
Case:

// Get the entire array (succeeds):

> db.foo.find({"key":"12345"}, {"array_field"})

{ "_id" : ObjectId("4c9a41204cb16aa4fe28c4cc"), "array_field" : [
        {
                "bar" : "data",
                "bam" : "more data",
        }
] }


// Attempt to get first element in array (index 0):

> db.foo.find({"key":"12345"}, {"array_field.0":1})

{ "_id" : ObjectId("4c9a41204cb16aa4fe28c4cc"), "array_field" :
[ { } ] } 

The second command fails to return anything. You can in fact use the $slice operator to get the element, but then the command becomes large and ugly:

> db.foo.find({"key":"12345"}, {_id:1, array_field:{$slice:[0,1]}}) 

When developing custom ORM classes to access Mongo, this special case will be needed when accessing an array element by index. This seems unnecessary.
I suggest that Mongo handle dot-notation the same for Arrays as it already does for Objects, and simply allow you to select by index. (Note, that a find query does allow array indexes in the dot notation, but the field select param does not)