Nikon D300s has some important improvement from Nikon D300 to stay competitive in the market. Most of the  improvement are exactly the same as rumored.

Here is the recap on what is improved according to Nikon USA site:

• 7 fps body only, 8 fps with battery grip
• Refined D-Movie function including an external microphone input for clear stereo sound recordings
• Dual card slots for CF/SD memory cards
• Quiet shutter release

What features remain the same

• Multi-CAM 3500DX AF sensor module featuring 51 AF points offers fast, efficient and precise autofocus coverage across the frame
• Large, bright viewfinder with approx. 100% frame coverage and approx. 0.94 magnification
• Nikon’s original DX-format CMOS image sensor; 12.3 effective megapixels
• Magnesium alloy body for light weight and rugged durability
  

Because most of the cameras released in 2009, has movie mode, Nikon have the pressure to release include it on the D300s. But the video mode might still not the best in DSLR camera world. First, it is only 720p @ 24 fps. Which is the same as Nikon D90 and Nikon D5000, while its competitor, Canon already ahead with manual exposure control in Canon 5D. Panasonic GH1 also is serious in video business and regarded as the best implementation of video mode control so far.

It is also interesting to see that Nikon stand still in megapixel race. D300s will still have 12 megapixel sensor while Canon 50D, the main competitor of D300 already at 15 megapixel. While big resolution is good for extra large print, but generally it make the photos noisier when zoomed 100%.

Nevertheless, improvement like rapid 7 fps, dual card slot and quiet release (first introduced by Nikon D5000) is nice. For Nikon D300 users who are shooting primarily in still images, the D300s might not look like a worthwhile upgrade, but for Nikon D90 or D5000 users or who likes to have movie recording feature, you might feel this camera is significantly better in most aspects.