Yep on my mtb. It doesn't slip but I have surly tugnut on it so Its not that suprising

I know people who use one with dremeled out vertical dropouts ala sheldon brown without any problem. Really the differnce in force the qr has to withstand between mostly horizontal droupouts and trackends is pretty minimal.

If it did slip on a brakeless fixie though it would probably do so while you where skidding so you may not want to mess around with it. chain tensioners are relatively cheap and are nice for adjusting tension too.

Incidentally tracknuts are used on the track mainly because qr could be dangerous if it hooked in someones spokes and you don't need to do quick tire/wheel changes anyway.