Observed behavior:
rosbag with --duration runs forever, and records a bag file for the entire run-time.

$ time rosbag record --duration 5 chatter
[ INFO] [1353036305.828513349]: Subscribing to chatter
[ INFO] [1353036305.831663013]: Recording to 2012-11-15-19-25-05.bag.
^C
real 0m22.536s
user 0m0.264s
sys 0m0.896s
$ rosbag info 2012-11-15-19-25-05.bag
path: 2012-11-15-19-25-05.bag
version: 2.0
duration: 21.6s
start: Nov 15 2012 19:25:06.07 (1353036306.07)
end: Nov 15 2012 19:25:27.67 (1353036327.67)
size: 23.1 KB
messages: 217
compression: none [1/1 chunks]
types: std_msgs/String [992ce8a1687cec8c8bd883ec73ca41d1]
topics: chatter 217 msgs : std_msgs/String

Expected behavior:
rosbag runs for roughly the specified time and then terminates.