2 examples with unexpected results:

db.test.insert({
point:[1,10],
tags:[

{k:'key',v:'value'}

,

{k:'key2',v:123}

]
})

db.test.insert({
point:[1,10],
tags:

{k:'key',v:'value'}

})

db.test.ensureIndex(

{point:"2d","tags.k":1,"tags.v":1}

)
db.test.find({
point:{"$within":{"$box":[[0,0],[12,12]]}},
"tags.k":"key"
})

Only 1 result:
{ "_id" : ObjectId("4d373f753ffe71e78a71bfc0"), "point" : [ 1, 10 ],
"tags" :

{ "k" : "key", "v" : "value" }

}

2)
db.test.drop()
db.test.insert({p:[1112,3473],t:[

{k:'a',v:'b'}

,

{k:'c',v:'d'}

]})
db.test.ensureIndex(

{p:'2d','t.k':1,'t.v':1}

,

{min:0,max:10000}

)
db.test.find(

{p:[1112,3473],'t.k':'a'}

)
No result.

Some discussion here: http://groups.google.com/group/mongodb-user/browse_thread/thread/8fa375ba78081b2c/fd9967f89ff758da