As of now update only acts on one object at a time, even though the query may match several. It is of course possible run update n times, where n is the number of objects to update, but that quickly becomes a bit tiresome. Would be nice if update worked more like remove, which acts on several objects.

http://groups.google.com/group/mongodb-user/browse_thread/thread/e7bfc0cdaca06419/8fcc1f5bd04fd614?lnk=gst&q=update#8fcc1f5bd04fd614