Can you please let me know if this works on OSX 10.5:
https://s3.amazonaws.com/gavinandresen-bitcoin/bitcoin-0.7.2rc2-macosx.dmg
I set up a new build environment on an old, 32-bit OSX 10.6 laptop, and I think I fixed the 10.5 compatibility issue.
(some MacPorts maintainers don't seem to be terribly good at making sure their ports honor BOTH the -arch AND the -isysroot settings...)