> var rt = new ReplSetTest(

{nodes:7, name:'slave_ok_no_master'}

);
> rt.startSet(

{oplogSize:200}

)
> rt.initiate()

Start up a mongos with rt.getURL() as a shard() and connect

> s='x'
x
> while (s.length < 128*1024) s += s; s.length
131072
> for (var i=0; i<1000; i++) db.foo.insert(

{s:s}

)
[takes a little while]
> db.foo.count()
1000
> db.foo.find().itcount()
1000
> db.getMongo().setSlaveOk()
> db.foo.find().itcount()
Fri Feb 4 17:18:30 Error: getMore: cursor didn't exist on server, possible restart or timeout? shell/query.js:104

It looks like the works/doesn't work threshold is 32/33 which is the same threshold where a the results won't fit into a single message:

> db.foo.find().limit(32).itcount()
32
> db.foo.find().limit(33).itcount()
Fri Feb 4 17:28:59 Error: getMore: cursor didn't exist on server, possible restart or timeout? shell/query.js:104
> Object.bsonsize({_id:ObjectId(), s:s})*32
4195264
> Object.bsonsize({_id:ObjectId(), s:s})*33
4326366
> 4*1024*1024
4194304

I added some debugging info into the code and it looks like the getMore is going to the master.