According to the Broadcom media release:
The BCM2835 is a cost-optimized, full HD, multimedia applications processor for advanced mobile and embedded applications that require the highest levels of multimedia performance. Designed and optimized for power efficiency, BCM2835 uses Broadcom's VideoCore® IV technology to enable applications in media playback, imaging, camcorder, streaming media, graphics and 3D gaming.
Low Power ARM1176JZ-F Applications Processor
Dual Core VideoCore IV® Multimedia Co-Processor
1080p30 Full HD HP H.264 Video Encode/Decode
Advanced Image Sensor Pipeline (ISP) for up to 20-megapixel cameras operating at up to 220 megapixels per second
Low power, high performance OpenGL-ES® 1.1/2.0 VideoCore GPU. 1 Gigapixel per second fill rate.
High performance display outputs. Simultaneous high resolution LCD and HDMI with HDCP at 1080p60
VideoCore chips can run complete applications - they are not simply video DSP chips that require a separate processor to supply and collect data. In practice they are often used like this, as companies usually prefer to cautiously assimilate new technology rather than take a big risk in porting a large amount of application code from an existing ARM based design. The Apple video iPod is a good example of this approach.
Low-power laptops use low-power processors and graphics chips, and therefore often struggle to play video at full frame rates. It isn't desirable or practical to port Windows onto a VideoCore chip, so only the video decoding need be offloaded onto a video accelerator board (e.g. using the the BCM70015 chip).
Blu-Ray players can also use it as a low-power video accelerator.
Noting that VideoCore chips were usually used with ARM based chips, the latest chips have VideoCore and ARM processors.
The reality of life is that the target market is stand alone applications not general purpose computing. The developers of the Raspberry Pi have configured the device as a low cost teaching aid. Don’t expect the Broadcom VideoCore to be open sourced - that will be committing corporate suicide.