We've definitely seen plenty of problem reports for MAMP, and lately there have been several with this issue.
Judging by the reports we've had about this, it almost seems like MAMP's PHP has some configuration settings that are (at least under or in combination with Lion) not allowing paths under /opt to be used.
Using a Mac as a development environment in general can be problematic, because Omeka officially supports, and the vast majority of Omeka installations run, on Linux. There haven't been many issues with this in the past, and we do our best to make sure Omeka works the same under Mac, but it's something to think about.
The different strategies for getting everything you need on Mac all come with different annoyances. For any of them, ImageMagick has to either be installed with MacPorts or Homebrew, which requires download Xcode and many supporting libraries, or getting the binary version and manually copying files to the correct locations. For many people it may be wiser to simply use a Linux virtual machine, which would also let you take advantage of the often-superior package managers provided by Linux distros, as well as allow you to move your dev environment with you across different machines and even platforms.