I setup a 2 shard cluster with auth. With a continuous query load of around 1000/sec the mongos process seems to keep chewing up memory until the OS kills the process:

Out of memory: Kill process 2567 (mongos) score 950 or sacrifice child
Killed process 2567, UID 502, (mongos) total-vm:8987492kB, anon-rss:7738860kB, file-rss:16kB

In this case I had upgraded the test server to 8GB of ram to make sure it would still occur and with 8GB it took right around an hour to use up all the memory and swap space before the OS killed it.