I have not been able to get my 60D to increase its shutter-speed faster than 1/250 sec when it's linked with my 600EX-RT. When connected (even when selecting the HHS option), it stops at 1/250. Not sure how to do this. :-/
I'm also the happy owner of my (first) 600rt! For the current price of €550 (wtf?!?!) for a *flash* it still feels somewhat silly, but I figured if I want to go into portrait photography all pros use radio control for off-camera flash, so better get the future-proof tech. And the flash sealing, build quality and features (aka 580ex2) are great. Finally: for the price of a Rebel, at least it's as large as a Rebel :-p
Since I'm also using the 60d:
* For on-camera flash, you should simply be able to use hss by selecting it on the flash, no idea why Canon made this step necessary at all. In Av mode, be sure to select "Auto" for the flash shutter speed range, *not* the default 60-250!
* For off-camera flash, hss doesn't work with the pop-up flash of the 60d controlling the 600rt. You need the 600rt to be the master on camera, and then all off-camera flashes (like my "budget" 430ex2) work with hss. Still, since the x-sync of the 60d is 1/250s, hss is only necessary for me for fill flash in bright daylight or capturing very fast movement.