This wasn’t documented anywhere, so I figured others might find it useful. In MongoDB you can tail a capped collection, much like using the tail -f command. This works because Mongo uses cursors. Tailing’s a nifty trick, especially if you’re using Mongo for logging.
I’m using Phil Burrows mongo_db_logger to log all Rails reqs to a capped collection at the moment, I’d recommend dropping this into your rails app, after modifying it to read your mongo config so you always have it ready.