ask laszlo:
http://bitcointalk.org/index.php?action=profile;u=143he is building the mac version and would know best.
imho try at least 10.5. i'm not a bitcoin developer but it seems to me i saw in the project development part of the forum the information that they are using some library for the GUI that requires at least 10.5 version of mac os. also you need intel cpu. bitcoin will not work on a power pc mac. but that is different from the gui problem that imposes the 10.5 mac os version,