What is with the iMac advice? The new 15” MBP is great for audio production. I run many instances and VST’s with countless effects and plugins with no stall or issues. The only reason why you’d need an iMac is if you are doing post-production with large video files. That’s where that extra 16GB RAM will help in production, but even then, the 15” MBP is plenty capable.
You’ll certainly want a quad i7, period. Most DAWs now take full advantage of multiple threads and cores.
I use Ableton, Logic, PT, Cubase, Slate Digital, NI Ultimate 11, Waves, Xfer VSTs, and many synths hooked up to an interface... It isn’t rocket science.
EDIT: I JUST REALISED HOW OLD THIS THREAD IS.
Yeah, time has passed, but it's still a valid theoretical discussion.
I think the MBP is a fabulous machine for audio production, if you need portability. I haven't worked in the business for nearly 20 years, but when I was recording concerts on location, I would have loved to record direct-to-Mac. It would have saved me the time of bumping my DATs to the Mac Quadra 950 back in the studio (running Sonic Solutions). There was certainly no way I would have taken the Q950 and a 21" CRT on location. At the time, I was fantasizing about the day when it would be possible to do it all on a laptop.
However, there's also no substitute for a large display, and for the extra storage and horsepower possible in a desktop. It's like comparing a compact/portable mixing board like a Mackie to a whopping SSL studio console. The difference in performance and capability is not quite proportional to size, but there are differences.
Now part of the Mackie vs. SSL thing is completely down to making an impression on the customer. If you want to justify a high rate, you need some sexy hardware to back it up. A 27" iMac is going to make the better impression, especially if flanked by an additional pair of Thunderbolt displays. ;-)