I saw a thread on Usenet tonight which mentioned that the class-dump utility didn't work with universal binaries, so I took a few minutes and patched it up. It only displays data for the host architecture side of the binary, but that's still better than aborting.
To get the changes, download the source code for the main project, then replace CDMachOFile.m with this version and build.
Update: Steve Nygard did a much better job than I did and not only added universal binary support to class-dump, but also added a --arch flag so you can display any architecture you like. class-dump 3.1 is now available.