This is partially for me in the future just in case I'm stuck doing this again, and it's partially for the lone googler in the future who comes by needing a quick fix.
So, the short version:
Getting MySQL
Grab the 64bit version from MySQL ("Mac OS X 10.5 (x86_64)"). Easiest way is to grab the DMG, use the installer, and then add the preference pane.
Setting up MySQL
It's probably helpful to add an alias for mysql in your ~/.profile:
alias mysql=/usr/local/mysql/bin/mysql
alias mysqladmin=/usr/local/mysql/bin/mysqladmin
Preparing for Ruby
Grab your Leopard DVD and install Xcode if you haven't already... it'll help you compile some of the native C code involved with the MySQL gem.
Install the gem
Rock this:
sudo env ARCHFLAGS="-Os -arch x86_64 -fno-common" gem install mysql -- --with-mysql-config=/usr/local/mysql/bin/mysql_config
That'll build your MySQL gem for you. Most of the issues with errors like "lazy symbol binding failed" are the result of not setting those architecture flags; something with Leopard pushing multiple architectures but MySQL expecting only one.
Those are the main sticklers; if I missed a step along the way it's probably easy enough that a quick Googling would solve. Kudos to http://cho.hapgoods.com/wordpress/?p=158 and its commenters in helping figure out some of those pesky details.
I tend to change the icon of my hard drive to reflect whichever Mac I'm on -- my MacBook has a little MacBook for its icon, and my iMac has an iMac for its icon. Usually I go to a third party who creates these clever looking icons and then add them to my machine, but with Leopard's gorgeous 512x512 icons I wanted to just use them and be done with it.
Unfortunately it's a little tricky. First, make sure your account has administrative access. Then find your icons. These are where the actual .icns files are located. (You can Cover Flow this folder for some nice eye candy.) You're not quite done yet; usually you can just copy the little icon in "get info" over and be done with it, but that only copies the icon for the ICNS file format.
Drag the .icns file over and then run this command in Terminal:
sips -s format png ~/Desktop/icon-filename.icns --out ~/Desktop/converted.png
(Note: "--out" should be two hyphens.)
Then right click on the newly created PNG file on your desktop, click "Get Info", and highlight the little icon above the Spotlight Comments section. It should turn blue. Do a command+C, then right click on your hard drive's icon, click "Get Info", highlight the small hard drive icon again, and command+V to paste over the new icon. It should change the desktop hard drive icon (if you display hard drives on your desktop, that is).
I feel this process got slightly more complicated with Leopard due to the addition of QuickLook, as there's almost two icons per file now: the regular extension-based icon, and the dynamically generated preview icons.
Interestingly enough, you should not attempt to install new operating systems on machines that have faulty logic boards. I began to realize this after my Leopard install failed about five or six times.
As a bit of backstory: a few months ago my MyBook died (theoretically), which led me on a crazy chase to figure out what the heck was going on with my Macs and hard drives. The second and third hard drive replacements worked over USB but not Firewire. Long story short, it seems that some cheap Western Digital Firewire cables fried the Firewire on my iMac, my MacBook, and the drive itself. Three replacement hard drives later I found this out, and promptly got my MacBook's logic board replaced to fix the Firewire ports. My iMac is currently in the shop getting the same treatment.
Unfortunately, it seems I got a faulty logic board for my MacBook as a replacement. Firewire ports work now, but two or three times a day it just powers down... no errors, no logs, nadda.
Naturally, an OS update would be prime time for those shutdowns to occur. So that happened a good five or six times until the one happily went through without a hitch, which brings me to the present: typing on a new Leopard install.
It feels good, too.
I couldn't resist; I had to go see Leopard's launch at the Apple Store in Shadyside. Free t-shirts, free water, waiting in line with hundreds of fellow Apple fans... it was a good time. Got to play around with Leopard for ten minutes or so, though I didn't take a copy home with me; I have to wait until tomorrow morning to get my enviable student discount at the bookstore.
