Hi, I posted a similar problem concerning this same video here:
http://www.mirillis.com/forum/viewtopic.php?f=16&t=63
It was a problem in Splash 1.1 which was related to how Splash handled 1080p videos with 16 reference frames.
This bug is now fixed in Splash 1.2, and I can confirm that it does play with GPU acceleration on my system.
If it still plays without GPU hardware acceleration, it most likely means that your graphics chipset does not support nVidia PureVideo acceleration of 1080p content with 16 reference frames (this is particularly true of older Intel GMA chipsets).
Whenever Splash plays a video on a system that cannot accelerate a particular sample it automatically defaults to CPU decoding.
If your system has a PCI-Express x16 slot, I suggest getting a cheap GT 2xx series dedicated graphics card with CUDA support (approx. $65). With that, you'll be able to play all available HD content with GPU acceleration through Splash.
Your other option would be to re-encode the video with a lower number of reference frames, but of course the quality will be slightly lower, though since the source is HD the quality loss probably won't be too noticeable.