Though TiVO's software is open source, any "unsigned" modified code is blocked from running on these devices. Home-brew DVRs act like TiVOs, with a few side benefits. The article mentions, incidentally, that this is not a project for the faint of heart.
My MythTV box has been humming in my living room just shy of a year. It's not a project for a new user, but it's a better application, and less complicated to install and maintain than you've been led to believe.