One of our test machines, a relatively standard IDE-based machine with 32M of memory and a Cyrix 6x86-PR166+ at 133 Mhz, doesn't boot the latest Starbuck or Rawhide boot images (either boot.img or bootnet.img). Syslinux reports 'boot failed' after printing its startup banner. (The same floppy will boot happily on another machine.) The same machine is happy with the RedHat 5.2 boot disk images (and RedHat 5.2, which it has installed). If you need more details about the hardware or have tests you'd like to see performed on the machine, let me know.
I attempted to verify this problem with a test lab system consisiting of a Cyrix P166+ and 32 megs of ram using the latest 6.0 bootnet.img. I was unable to get the install to fail as reported.