Well, the current Apple one button is just about the most perfect mouse you can buy, period.
There is a UI argument being made, we know it well. Let's skip, since no one can legitimately deny the positive effect of a one button UI orthodoxy.
Then there is the argument about speed and ease of use: this is the one in which ALL of the multibutton proponents don't have a fvcking clue.
Scroll wheels are the most ergonomically evil invention ever to marr the computer mouse. They are absolutely horrid, and there is a mountain of evidence to tell you why. Don't forget, that on the keyboard we have a a much faster and far more ergonomic way to scroll -- THE FVCKING ARROW KEYS!!! If the ludittes would get their sweatty paws off the mouse long enough, they might realize that using the keyboard properly is faster and safer than trying to turn a mouse into an advcanced control surface.
Which "pros" really use a multi-button mouse?
Musicians, artists and editors all use specialized control surfaces when dealing with apps that require more than one button. Whether we're talking about mixing boards, tablets, shuttle pros, coded keyboards, we aren't talking about multi-button mice. There is the case of shake, and ONLY of shake, and even that could be changed...
As a mechanism for UI control, a one button mouse plus keyboard remains the healthiest (most comfortable), quickest, and most elegant interface.
All that said, if Apple were to change, I wouldn't look for a traditional 2 button mouse.
Firstly, any mouse would have to be ambidexterous: it would have to seamlessly recognize a left or right handed operation, and change the mouse pattern accordingly, that, or 2 buttons would have to be arrayed in a way that represented 'main' and 'auxiallary', rather than 'right' and 'left'
Secondly, only the hightest ergonomic principles could be expressed in the design, that negates any form of conventional scroll wheel, as they are all failures from a biomechanical standpoint.
Thirdly, the ergonomic advantages of a whole hand over one button would have to be re-established by a superior ergonomic shape -- difficult... Most of the "ergo-look" designs aren't all the ergonomic. Turning them on their side, to avoid twisting the wrist is too ugly, despite the benefits.
Apple's latest two mice have been able to mitigate this problem (and remain attractive enough to leave on a desk) by forcing a grip that reduces fine muscle movements (no single finger control) and either through the round shape (of the puck) or the side tabs (of the current design) force users to lift their wrists slightly and use a larger muscle group in controlling the mouse.
Is there a shape that keeps the inherent ergonomy of gross muscle movements, and supplies two buttons and a scroll interface?
Maybe a headlight-like shape! Think breast implants!
A half spheriod cone with two or three rings. You put your hand around it less like a mouse and more like a casually held football -- one that you're about to toss through the bottom of the desk. Whether you grip it left or right, your index finger falls nearer the top (on the first lace) and your middle finger falls nearer the base. Also whether you grip left or right, your wrist is turned at least 45 degrees from horizontal, and maybe closer to 50-60 degrees depending on the conical shape used, again, much better.
Scrolling? Well, though I believe this best accomplished through a form of gestural recognition, one could at this point employ some sort of iPod tech at the top of the cone. You scroll as if teasing the areola of this rather breast-like shape.
We've had the puck, now comes the tit! Think of a airport base station, smaller in circumference, but taller, curved in slightly at the base to allow the pinky to slip under more comfortably.
I guarantee you that's what Apple will release, if/when they go multi-button.