Building your own game console is entirely possible. I researched this about a year ago. Unfortuantly, I haven't done much with it, but it's still a major possibility. There's a couple routes you can take. The XGameStation might be a good one, one I might make to learn a little more. You could also look into building a cheap pc, packing it into a tiny case, and install a stripped down version of linux. I don't know much about the subject, so I won't ramble, but I believe anyone who wanted to, could build something of the equivelant of a Nintendo in less than a year if they put time and thought into it.

:)

[EDIT] Here's my old thread that may help you out.
http://www.gamedev.net/community/forums/topic.asp?topic_id=261360