Granted, this may only be Nikon's second foray into a video capable DSLR (the first being the D90), ardent Nikon fans would no doubt recall the rolling shutter issue the D90 suffers from. The D300s unfortunately exhibits the same problem.
To quickly recap, the D90 suffers from a jelly-like wobble every time you pan the camera (that is, moving the camera sideways from left to right, or vice versa). If your subject has straight, vertical edges, you can see these edges and lines bend and sway. For the football fans among us, you could say it mimics David Beckham's famous free-kicks. If you were to treat the video functionality as a bonus function, this might not be too bad of a concern for you.
Now, you would think that Nikon would have a fix for this, especially since the D300s is the second DSLR in their lineup to feature video (though also being the first semi-pro Nikon camera to do so), this doesn't appear to be the case. With the professional-level D3s hitting the markets soon, and with it too possessing video capabilities, we can only hope that Nikon fixes this problem soon. Or Nikon could remove it entirely and stick to what it is they do best - great looking stills.
You can find two great videos here taken by professionals prior to the launch of the D300s. Both videos were taken off Nikon Australia's YouTube channel:-
Do note that these videos are to be considered as professional works, and results will vary from person to person. If you didn't manage to notice any problems with the video, try looking at around the 10-second mark for the second video - look carefully at the rotor blades as the helicopter takes off. Also, be sure to check out the comments in the respective videos to see what other 'Internet critiques' had to say about the video capabilities of the D300s.