Getting RMagick, ImageMagick, and all of their dependencies downloaded, installed, and running correctly on OS X has generally been a headache - the exception to the rule that Rails and OS X 'just work'.
I spent several days back in 2006 getting RMagick working, and recently spent an afternoon redoing everything when I upgraded to Leopard and all of the Ruby gems got 'helpfully' moved for me.
I've also just upgraded from my somewhat worn out PowerBook G4 to a new Core 2 Duo MacBook Pro - and of course RMagick doesn't work again. So instead of doing several hours of billable work and making actual money, I got to poke around the depths of my system and do a lot of Googling to get things back to the way they were before.
This time it only took about two hours - only! - and here are some helpful hints if you're wrestling with RMagick on an upgrade from PPC to Intel on Leopard:
If you've already got a non-functional version of ImageMagick / RMagick on your system, the magic one-click 'rm_install' script won't work. Instead, you'll need to do some typing and copy-and-pasting from this handy onrails.org article. Note that instead of 'wget' you might need to use 'curl -O' instead. Also instead of 'make' you'll want to do 'sudo make' in most cases. Your mileage may vary depending on if you have a separate admin user or not (I do). Some of the urls used for getting the libraries are obselete now, but you can google for the filenames and find an alternate source pretty easily. Once you've downloaded the original archives, don't delete them! - since you'll probably need to do the installation again at some point and it's easier to just delete the install directories and re-expand the archives than it would be to hunt them all down again.
The last line in the script is 'sudo gem install RMagick' - this is wrong. You actually want to follow Greg's advice and type 'sudo su - ' to get root, and then type the magic (without the stupid 'k') incantation:
ARCHFLAGS='-arch i386' gem install rmagic
That seems to work. If it doesn't, block off another few hours and try again!
I find it amazing that the transition from G4 to Intel was almost completely seamless - all my data and configuration copied over a few hours with a Firewire cable. Almost all of the software worked perfectly without any hassle, even though the entire architecture underneath had changed. Making the transition that invisible is actually very difficult - and it's a tribute to Apple that it was almost invisible. Things like RMagick remind us of how messy software can usually be.
Hopefully the latest round of upgrades has ended for a while and I can actually get some work done for a change.