[CentOS] CentOS 5.4 32-bit vs. 64-bit strange mystery WRT PXEBoot

I am in the process of migrating my desktop system from 32-bit CentOS 4.8 to
64-bit CentOS 5.4.  One of the things I want to be able to do is boot my
IBM Thinkpad X25 laptop via its PXEBoot ROM over the LAN (it has a hard
disk and boot locally just fine, I just want a way to boot it for rescue
and/or new O/S installs).  On my 32-bit CentOS 4.8 this works just fine.
It also works on another machine, running 32-bit CentOS 5.4.  On my
desktop system running the 64-bit version of CentOS 5.4 it does not. 
The DHCP part works fine, but I get a 'TFTP ARP TIMEOUT' message.  The
various config files are all the same (or consistent). I cannot figure
out what is wrong.  If I let the laptop boot up normally, it is happy to
use tftp to fetch pxelinux.0 from the 64-bit CentOS 5.4 system.  It just
won't fetch it from the boot ROM.  What am I missing here?

-- 
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