Having failed to find an OpenGL card with SDI output is there one with YUV output about anywhere?
Cas 
Most cards have TV out these days. That’s YUV, although it’s 4:1:1 (if it’s composite) or 4:2:2 (if it’s S-Video).
If you want actual pro quality, you’d have to pipe the RGB out (in DVI format, preferrably) into an external converter box. That’s likely to not only give you better quality than anything anyone could put on a card, but also to be instantly available, not require changing your software, and probably port better to future set-ups.
Yes, you’re right, tv out is absolutely no good as this is for pro stuff (live tv). We were going to use a scan converter but they’re kinda expensive.
Mind you, there’s still the problem of creating a key from the output signal automatically anyway. So I suppose a scan converter it is.
Do you know of a scan converter that can turn DVI-I (such as wot comes out of an ATI Radeon for example) into SDI with a key?
Cas
Nope, sorry! Check the trade mags, or wait for the next NAB exhibition 
Sadly NAB comes just after when we need to go on air 
There are some truly awesome cards waiting in the wings which are allegedly going to demo there.
Cas
I wouldn’t write off the S-VHS output just yet. It may be consumer quality parts, but with a little clean-up (especially of timing and color burst) you might be able to crank it out, at least until a better cheap solution comes along. There are boxes designed to do just this, for airing VHS tapes people send in and stuff like that. Might be cheaper than a scan converter with DVI in (if you can even find one :-).
Would a VGA to Component Video (Y,Pb,Pr) converter be of any use?
You can get one of these for $129 here:
http://www.audioauthority.com/aacconsumers/9a60detc.html
Oh, there’s no chance of getting away with SVHS now that the whole studio has just gone digital. The scan converter’s really a last ditch option which has an SDI output on it (and key generator, so I can’t use that cheapo device).
FYI the scan converter we’re looking at is a top-of-the-range Snell and Wilcox. £90 a day to hire it or about £9k to buy. Studio tests are on Friday. I hope it goes well as my last year’s (unpaid) work depends on it, and I’m pretty skint now.
Cas