Do you guys use IS on yer lenses if your flying them with Glidecams, Steadicams, or whatever stabilizers your using?

Generally, no. Stability should be attained through technique with the rig.

The IS lenses can be "fooled" by the motion of the Steadicam, causing an undesirable floaty quality...or worse, a sudden "jerk" when the IS mechanism gets confused.

I was wondering this as well, whether it's better to turn the IS off as with tripods, or leave it on just in case.

Turn off the IS when on any stabilizing device. As mentioned, They'll compete and cause stuttering. If the GlideCam footage isn't as smooth as you'd like, mild motion stabilizing in post does wonders. There isn't a way to repair a stutter if the IS decides to kick in at the wrong moment.

Not all IS systems are the same. You cannot compare let's say Sony EX1 and AX2000. EX1 has a "weaker" IS as it's aimed at a more experienced user. I can say that I never saw it's IS interfering or doing anything weird when working with steadicam (Pilot in my case). I usually keep it turned off but I did work in some situations where it was a real saver.

Working with AX2000 on the other hand was completely different, I had to disable IS as it was too aggressive for steadicam.

there is a terrible stuttering if you'll shoot EX1 on steadicam slow motion with IS on, other than that i never noticed any difference, but it could be different for other cameras;