Currently my log file sits at 32 meg. Did I miss an option that would split the log file as it grows?
Using logrotate is a good option. while, it will generate 2 log files that fmchan commented, and you will have to follow Brett's suggestion to "add a line to your postrotate script to delete all mongod style rotated logs".
Also copytruncate is not the best option. There is always a window between copy and truncate. Some mongod logs may get lost. Could check logrotate man page or refer to this copytruncate discussion.
Just provide one more option. You could write a script that sends the rotate signal to mongod and remove the old log files. mongologrotate.sh is a simple reference script that I have written. You could write a simple cron job or script to call it periodically like every 30 minutes.