Chris, I own an X10, an X-S1, and the EF20 and EF42 flashes (I will round up an EFX-20 soon). Not too concerned with slave setups yet, but I am planning to get the Canon off-camera cable (you can also do the same thing with the Velo-brand Canon o-c cable, either available thru B+H or the other usual suspects).
This way, I can maintain full TTL control, which is what I want: The Fuji flash implementation is fabulous. I once did a test on a close static display, and the results from the X10's built-in, the EF20, and the EF42, were indistinguishable. As far as I know, there are currently no slave setups that carry thru this TTL operation. Anybody know of any? All I have found thus far is other-brand automatic flashes tripped by an on-camera flash from the camera. One of my favorite flash techniques is to hand-hold the flash off the camera (up high, to one side, etc.), so you can see my interest in the Canon cable.
_DSF0358 by jhawker, on Flickr
I have used the x10 and now x100 with off camera flash.
This one from late last year was done with the x100.
The flash was a canon 540 ez shot through a Profoto translucent umbrella.
Cybersync triggers and receivers.
While either of the cameras will synch at high shutter speeds I didn't need it here.
I use my cheap chinese ebay radio triggers and my old Nikon SB-28 flash. It's obviously going to be a manual setup without TTL support, but it works well. I even used the triggers to fire my pair of Elinchrom Style 400BX monolights and with the high sync of the X10, it's awesome (thanks leaf shutter)!