After having logged slow queries (>3 sec) I observed a few queries which took about 1271303044 ms (+- 5000 ms).
Could it be that these queries were executed with a noTimeOut option, so they would live "forever" (max ~1271303044 ms) on the mongo server?
Could it be an (unintentional) overflow of some mongo counter which results in termination after around 1271303044 ms?

According to mongo, the longest query took over 14 days (1271309097
ms):
{
"_id" : ObjectId("4f7dc7dc559a38abe85a91cd"),
"ts" : "2012-04-05T14:24:59Z",
"op" : "getmore",
"ns" : "offerStore.offer",
"query" :

{ "missingSince" : null, "shopId" : 5040 }

,
"cursorid" : NumberLong("1817499255547817100"),
"nreturned" : 68132,
"responseLength" : 3883544,
"millis" : 1271309097,
"client" : "#.#.#.#",
"user" : ""

}

Is this reliable?

The system time of mongod and mongos did not change that much. They are synced by ntp.

Related forum threads:
http://groups.google.com/group/mongodb-user/browse_thread/thread/6323765b69e79208
http://groups.google.com/group/mongodb-user/browse_thread/thread/7378a4610f1c9bf2