Andrew, can you have a look at installing SilverStripe on a Mac when using [url=http://www.entropy.ch/software/macosx/php/]Mark Liyanage's PHP package for MacOSX[/url]? I was talking with someone yesterday who was running into installation problems to do with PHP settings.
I don't think it has as high a usage as MAMP, but any improvement done to the installer to solve the problem would presumably fix problems experienced by others, too.
By the looks of, the major possible problem would be that it configures php.ini with short_tags disabled. Other than that, check the memory limit...
I wonder if a possible fix is just to include an .htaccess file with PHP flags fixing the above, and people can manually chuck that stuff in? You could even consider putting these sorts of flags in, uncommented? (I don't know if I'm happy with an .htaccess file with a whole bunch of directives in there already. What do others thing of that?)