In mongo docs is written that "In MongoDB 2.2, compact blocks activities only for its database." and "You may view the intermediate progress (...) by running the db.currentOp() in another shell instance."
http://docs.mongodb.org/manual/reference/command/compact/

That's not true when authentication is enabled because you can't authenticate against the admin database and you can't run db.currentOp() without being authenticated.

$ mongo s230:27017
MongoDB shell version: 2.4.8
connecting to: s230:27017/test
> db.currentOp()
{ "ok" : 0, "errmsg" : "unauthorized" }
> use admin
switched to db admin
> db.auth("admin", "xxxxx")//this blocks while compact is running!

This is quite annoying because we can't check the server status while compact is running.
Btw. MMS shows the server also (wrongly) as DOWN while compact is running!