I've shot with the studio lights and a 50D before, and I've shot with speedlights and the 1Ds MkII before, but I just did a shoot with the 1Ds MkII and studio lights...
... and I noticed that at a shutter speed of 1/250 I was seeing the rear curtain.
So I just had a look in my manual and behold:
The EOS-1Ds Mark II can synchronize with compact, non-Canon flash units at 1/250 sec. or slower speeds. With large studio flash, the sync speed is 1/125 sec. or slower. Be sure to test the flash to see if it synchronizes properly with the camera.
Hmmm... I didn't know that before! Any idea why this is the case? Do large studio flash units take longer to fire??
PS: While searching, I noticed 'high-speed sync,' then read all about it here: http://www.ausphotography.net.au/for...nc-Flash-Canon - great stuff, I've got to try that!