Note: as of Feb 2012, hikers can no longer hike or access the area underneath the arch, due to Native American cultural considerations. As of March 2012 the Forest Service has erected a wooden barrier fence both at the base of the arch and atop the arch in order to prohibit visitation under or atop the arch. It is still possible to hike a nice loop in this area, just without the under-the-arch portion. The Googearth map below shows the approved route nowadays. Currently there are no signs whatsoever showing or directing you on this route, but the well-worn trail is fairly easy to follow.
The Catherine Creek Arch, as seen from below it on the loop trail:
The hiking loop I describe takes you underneath and up atop the arch. This route is slated for closure by the Forest Service due to Native American cultural concerns. Go visit the arch while you still can, before it gets fenced-off!