I was hoping for collection-level locking to be a part of the 2.4 release. I didn't see any mention of it in the release notes. Last I heard they were going to implement collection-level locking and then begin work on document-level locking. I'm still hoping document-level locking isn't too far off.
Collection level locking isn't in 2.4. Working on more granular locking for 2.6/2.8. May skip collection level entirely for something more granular. More details can be found in the collection level locking ticket https://jira.mongodb.org/browse/SERVER-1240
From an atomicity perspective, yes. From a performance perspective, no; other concurrent operations affecting anything else on the whole database wait on your write.