I received this tip anonymously - I have no idea if the described issue is real or not (my D300s is still on order). Please take this with a grain of salt until we get some real reports from the field:
D300s movie recording issue
Apparently D300s has few issues with movie recording. below is a Q&A written by Nikon to distributors.
Q1： During movie shooting, recording stopped before the count down display reached “00m 00s.” Does this indicate damage or a defect?
A1：We will need to inspect your camera. Please contact an authorized Nikon service representative.
Q2：Why does this issue occur?
A2：Movies may not be recorded when external microphone is used and free space on the memory card is running low.
Q3：Is there anything that can be done to prevent this from occurring?
Nikon service representatives will make adjustments to your camera.
Q4：What adjustments have been made?
A4：The length of movie-recording time displayed before shooting has been adjusted.
Q5：What changes can be expected when adjustments are made?
A5：With same free space on a memory card, the length of movie-recording time may decrease after the adjustments.
Q6：Under what conditions does this issue occur?
A6：We have confirmed that it may occur when movies are recorded at a frame size of 320 X 216 with external microphone.
Can someone confirm if such warning was sent from Nikon to distributors?