So I got me some Cybersyncs and low and behold, they sync to 1/1000th on the 1D. I thought the 1D could only sync to 1/500th (and does limit itself to that when the flash is in the hotshoe), but I've seen no degradation right up to 1/1000th. At 1/1600th the entire image gets a bit darker and at 1/2000 it's totally black though.
I tried it on my 20d and it'll go to 1/320th with no ill effects (up from 1/250), any more and the black line creeps in. Just wondering if anyone normally syncs to this when shooting with Cybersyncs and if it can be done with PWs. Here's a sample shot from the 1D.
Carl
There is a post over on the AB thread where he is syncing at 1/2500 (although at that speed you are starting to cut into the flash exposure time, so not getting all of the light). I use the 20D as 1/320 with no issue like you and I suspect they put some buffer on the speed it can sync at.... although apparently the 5D is not one of those with a buffer. Not sure about the others.
I just picked that monitor up and the box it came in was the only thing I could think of to shoot that had a good representation of white and color. Looking at it again, I guess it does look a bit like an ad.....just a pic of a box though.
I did a similar test and with an off-shoe hotshoe cable, I was able to get it to reliably sync at 1/2500, and occasionally 1/3200 while other times it went somewhat dark. Here's the link.
It sounds like the sync speed limitation you found is in the Cybersyncs themselves not being able to sync faster than 1/1000 reliably... not that there's anything wrong with that.
Out of curiosity, what happens when you shoot at ISO 3200 with a fast lens wide open and a full flash pop when shooting at 1/2000 or faster? Do you get an image visible or is it still totally black?
Yeah, I wonder what it is that's being captured. Technical sensor sync limitations aside, perhaps the camera is capturing UV or IR? Maybe just one color channel?
Could you try manually adjusting the WB on a RAW image of that shot and seeing if the colors of the birds display properly?
Here's my (totally unprofessional) look at what's happening.
For some reason, there's something unique and special about the red channel in the 1D. Many of us have raved about how wonderful the reds look coming out of this sensor.
For whatever reason, when you go past the sync speed, the red channel is still faintly able to detect light. There is also an ever-so-slight ability for the blue channel to appear as well, thus giving us the appearance of magenta. (Red + blue = purple. Red + a little blue = magenta... i believe).
We can see this when we WB to the magenta, neutralizing out the magenta cast. The red is gone, but there's still a hint of blueness left. The green is totally zapped out.
Why? Who knows. Looks kinda like an IR image though.
k, just for kicks, I went out w/ the 1D, 580ex off-shoe w/ the secondary E-TTL pins gaffered off, 100mm macro, and went around flashing the garden. These were shot at ISO 800-1600, f5.6-f11, 1/16000, flash very close to the subject at full power. Auto WB.
After twiddling around with the RAW WB a bit, it seems like it's effectively a B&W image with a very strong magenta cast. Shifting towards green doesn't really fix the color cast though. Basically you can apply any color cast you want, similar to sepia I suppose.
Very interesting, guys! This is very cool stuff you discovered. (okay....I'm sure you're not the FIRST to discover this, but I haven't seen or heard anything like this before)
Speaking of interesting, I just peeked in at one of the images at 100% to do some NR and noticed a weird digitalish pattern in the images. This is a 100% crop of the bottom left-hand side of the 4th image. The grain isn't like anything I've seen before.
ISO 800, -0.85 stops exposure compensation in post.
I just picked that monitor up and the box it came in was the only thing I could think of to shoot that had a good representation of white and color. 
Looking at it again, I guess it does look a bit like an ad.....just a pic of a box though. 