You can have the same concern with circular needles as with your straight ones -- that your stitches may slip off. Of course, you can use just regular point protectors for both, but the designers at Clover have developed an ingenious accessory to keep your loops on your circulars and your needle tips firmly and securely together all using one device.
The "Point Protectors for Circular Needles" are thumb-shaped with two holes in one end to insert your needle tips safely. These protectors come in 4-piece sets and are available in 2 sizes designed for different sized needles: Small for 0 to 8 U.S. (2-5mm) and Large for 8 to 15 U.S. (5-10mm). Get both sets so that you'll always be prepared no matter what size of circulars you're using.