I bumped into this myself recently so thought that I'd put a post out to let others know...
Historically, ATI has always had crappy (if any) TV-out support on Linux. Thus, any time I'd put a box together that needed to pump output to a TV, I'd been putting nVidia boards into them.
After reading a release announcement for the Xorg Radeon drivers this last week, though, I pulled out one of my old ATI boards and dropped it into my MAME cabinet. S-video output worked
like a charm. A far cry from last time I tried the board (~3yrs back) when the TV-out support was basically nonexistent (I was aware of the Gatos project, but could never get it working on my ATI board).
And, pleasantly enough, the ATI board (Radeon 7500) gave superior S-video output than the nVidia board (MX/400) I was using.
Thought I'd let others know; if you've got a Linux box and are trying to pump output to TV, we've
finally got some choices for what video card you want to drop into it.