The dream box is a nice solution, but even although it is based on the IBM PowerPC it does all the MPEG/DEmux/SI and all stuff in hardware.
If you want to do it yourself entirley in software for free then many projects are around the web for this, but you will find hard time to get Linux + DVB running good, I have seen problems even on fast PC with these cheap demodulator cards sold at auction sites.
If you are looking to design a STB then most vendors support Linux on at least one platform such as IBM, STM, and I think Connexant, Zoran and Micronas too.
:R