Here is a small console application to reproduce the bug

namespace ConsoleApplication1
{
class Program
{
class A
{
public ObjectId Id

{ get; set; }
public string Name { get; set; }

public int counter

{ get; set; }

}
static void Main(string[] args)
{
var a = new A()

{ Id = ObjectId.GenerateNewId(), Name = "Test1", counter = 1 }

;
JavaScriptSerializer json = new JavaScriptSerializer();
var s = json.Serialize(a);
var b = json.Deserialize<A>(s);
}
}
}

a and b are different. b does match Name and counter field but Id are different.

A.Id = 4fc68f495669bd1ba000e831
B.Id = 000000000000000000000000

s = {\"Id\":

{\"Timestamp\":1338412873,\"Machine\":5663165,\"Pid\":7072,\"Increment\":59441,\"CreationTime\":\"\\/Date(1338412873000)\\/\"}

,\"Name\":\"Test1\",\"counter\":1}

FYI, python serialized ObjectId looks like this:
>>> json.dumps(d, default=json_util.default)

'{"_id":

{"$oid": "4fc68f495669bd1ba000e831"}

, "Name": "Test1", "counter":1}'

Do you think JSON serialized ObjectId can match between drivers? (GUID JSON output are also different between drivers)