It's yet another idea for an extention. Just a boolean state specifying what the program would like.

You could try and rig some system dependant thing by talking to windows directly but I would doubt you'd have much luck since things like retrace syncing are in directdraw and you can't use directdraw with OpenGL windows (because some OpenGL drivers use directdraw themselves)

There's an extension to turn it off. Well, to hint at its state really. It doesn't seem to be very well documented. Here's a link I found to a page on the MESA3d site, which seems to be down.
http://www.mesa3d.org/brianp/sig97/exten.htm

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Scott Franke [druid-]
sfranke@usc.edu
druid-'s GL Journal
http://www.gamedev.net/opengl