Do you plan to support regular expressions in $in queries ?
> j =
{ name: "mongo"};
{"name" : "mongo"}> db.things.save(j);
> db.things.find(
);
{"_id" : ObjectId( "4ac3312ffce86d635643cb7c") , "name" : "mongo"}> db.things.find({name:{$in: [/.*mongo.*/i]}});
>