MongoDB (2.4.8 and 2.5.4) does accept the float `4.5` as an ordering:

```
> db.foo.ensureIndex({x:4.5})
> db.getLastErrorObj()
{ "n" : 0, "connectionId" : 92, "err" : null, "ok" : 1 }
> db.foo.getIndexes()
[
	{
		"v" : 1,
		"key" : {
			"_id" : 1
		},
		"ns" : "test.foo",
		"name" : "_id_"
	},
	{
		"v" : 1,
		"key" : {
			"x" : 4.5
		},
		"ns" : "test.foo",
		"name" : "x_4.5"
	}
]
```