Just noticed while copying a src.rpm from a Linux x86 box to a FreeBSD x86_64 
box:

root@fbsd-test:~# rpm -ivh rpm-5.0-0.20071015.1ark.src.rpm
Preparing...                ########################################### [100%]
        package rpm-5.0-0.20071015.1ark is intended for a i686 architecture
        package rpm-5.0-0.20071015.1ark is intended for a linux operating 
system

Using --ignorearch --ignoreos unpacks it, but all files end up in the current 
directory except /usr/src/rpm/*/.