I thought of having a play with E-UAE et al today on my shiny new MacBook, but decided to try something a little bit wackier instead. A Core 2 Duo 2GHz MacBook, running Windows XP in Parallels, can run the latest WinUAE at full speed as long as you drop the sound emulation down to 32KHz and change it to "Emulated" instead of "Emulated, 100% accurate".
EDIT: Hmm, the slowdown actually seems to be related to the GUI - open the GUI during a game with sound and the game will stutter until you disable sound. At that point, everything works OK again.
There are, naturally, a few problems. Once WinUAE is running, you cannot change any settings - F12 brings up the OSX Dashboard instead of the WinUAE GUI. That means you cannot change disks, etc, but isn't too much of a problem if you have everything on a hard file. F10 and F11 are also intercepted by OSX, so you might find that you have to change the WHDLoad quit key.
EDIT: Altering the Expose hotkeys fixes this.
Running Parallels in windowed mode seems to work with the screen differently than when it is running in full-screen. If you run the WinUAE pixel mode detection whilst in full-screen, switching to windowed mode will make WinUAE's display turn blank. The same is true of switching from windowed mode to full-screen.
The only way to quit WinUAE is to bring up the task manager and close the process, as the standard Windows close icon doesn't work, and you can't get to the GUI.
Finally, the mouse pointer goes a bit wacky. With an out-of-the-box Workbench 3 disk, with standard settings, just touching the trackpad on the Mac makes the pointer zip from one side of the screen to the other. Changing the mouse speed down to its lowest setting is still far too fast. One way around it is to use the cursors instead of the mouse, but that relies on you having stretchy fingers - to use the right-mouse menu, you need to put two fingers on the trackpad (for right-mouse emulation), click the trackpad button, hold down the left Apple key, hold Shift (to make the mouse movement quicker), and move the mouse with the cursors. Six fingers at least - I hope you're dextrous!
EDIT: Ahh, you can change the mouse speed in WinUAE. That's the pointer problem fixed in Workbench, but it's still too twitchy to play something like Lemmings with the trackpad. Might work better with a real mouse. I find that changing the mouse speed from 100 to 1 works best.
Anyway, I'm going to download the latest beta of Parallels with Coherence support, and see how WinUAE works with that.
EDIT: It sucks with Coherence - as soon as the OSX pointer moves off the Windows display, it disables the mouse. It works much better with Windows in full-screen mode. And in the currently-released version that supports resolution switching, not the RC for Parallels v3, which doesn't switch the monitor resolution.
Screenshot of Parallels WinUAE: