I'm getting this one too, even when I run "nagios -v" with the sample config file, so it's not to do with any single service message.
Linux sgiadm01 2.6.13-15.8-smp #1 SMP Tue Feb 7 11:07:24 UTC 2006 x86_64 x86_64 x86_64 GNU/Linux
SUSE LINUX 10.0 (X86-64)
VERSION = 10.0
Nagios version 2.0
Maybe to do with compiling on 64 bit machines?