If you use a Mac as your development machine, sometime soon you will probably install Snow Leopard. Set aside a day for the resulting PHP and MySQL issues. Some people report no issues with moving to Snow Leopard, so you may be lucky. I do not use MAMP, so I can't comment on that route
The good news is that it comes with PHP 5.3 including GD so you can use the standard PHP. The bad news is that you will need to reinstall MySQL. This means moving your data from the previous version to the new one. See Marc Liyanage's web site if you haven't done this before:
It was all working for me until I tried to import an SQL file from a Linux production server - it just would not work. In the end the fix was to add the following line to etc/my.cnf:
lower_case_table_names = 2
This is supposed to be the default in Mac OS X but went it astray somewhere. (either because of the Snow leopard upgrade, or my earlier attempts to 'fix' the problems.)
Hope this helps other Mac/SilverStripe users.