Looking at the speccynext web site, they moved away from using the Raspberry Pi to provide hdmi output, and now seem to have the hdmi connector directly on board and driven by the FPGA.
They still retain the 40 pin connector though, as the pi could be used for all sorts I guess...
There are some hurdles with hdmi - there are some open cores demonstrating basic video output on hdmi, but the technology is licensed out, so if you want to use ip cores from fpga vendors it gets expensive fast. The other issues are signal lengths, and board layout - there are some design requirements for hdmi, and cobbling together something that looks ok, may not actually work very when in practice.
I played around with the Terasic GX starter kit, which includes hdmi hardware - but beyond some basic video bars and patterns, it's a lot of work. I like my lego in bigger chunks