I would agree that multiple monitor support would be a really useful addition to the product. I'm also a Mac user and regularly use Aperture, Photoshop CS3, LightZone and Lightroom user and I run a setup with 2x 30in screens and an additional Cintiq for tablet work.
One mode where Lightroom would be significantly better to use would be an option under library mode to render full screen images on the 2nd monitor as you select individual thumbnails in grid mode.
Being able to relocate the tools would be useful too although I do see that this is a significant design philosophy change for the UI.
The other productivity enhancer would be the ability to render views in develop mode at different zoom levels on each screen, with the ability to straighten, crop, spot or locally select develop settings on any screen and simultaneously update the others. I can do this in PS and Aperture and it makes the Cintiq tablet a superb addition for more delicate work.
I don't think it's necessary to tear the whole UI apart to support these types of features in a future Lightroom version. The fact is that multiple screens are now very common configurations for folks doing image processing. It's also a very common configuration for laptop users these days with secondary larger screens in the office.
Heck, as a software developer I've used multi-screen configurations for too many years than I care to remember. The benefits of this configuration definitely isn't lost on the developers themselves, perhaps it's just the product marketing/product management folks who need punt up the rear to understand that this isn't a nice to have for many of us but is now an established must-have.
So, come on folks, how about it?