If you travel much with a carry-on case, then you'll appreciate an equipment bag that has 4 wheels as opposed to just two. I myself use the "Think Tank Airport Roller" (found here) quite often. LowePro has recently offered up a new Photostream SP 200 Roller Bag. The Photostream is a less expensive option if you're looking for a dedicated equipment case on 4 wheels that also works with carry-on size restrictions.
Lowepro SP 200 Roller Overview
The PhotoStream SP 200 has exterior measurements of 13.8 x 8.8 x 21.7" and weighs 9.7 lb when empty, matching carry-on requirements for most American airlines. For keeping small items such as cables or lens caps, two zippered mesh pockets are conveniently located on the lid interior and allow for easy access to your accessories. As for transport, the case comes with a three-stage telescoping handle and four 360° wheels for easy rolling. When hand carrying is needed, you can use the integrated handles found at the top, side, or bottom of the bag.