It should probably have a timeout or something? Return failure? I started up 1 mongo instance and then ran:

~/gitroot/p> ./mongo
MongoDB shell version: 1.3.5-pre-
url: test
connecting to: test
type "help" for help
> db.runCommand(

{getlasterror: 1}

)

{ "err" : null, "n" : 0, "ok" : 1 }

> db.runCommand(

{getlasterror: 1, w:3}

)