The Yamaha YZF-R6 has this...
The Yamaha YZF-R6 has this nice arrangement to ensure that the throttle butterflies never open more than the twistgrip is turned.
Over the past few years we've seen motorcycles becoming increasingly loaded with electronics and rider aids, for both safety and performance. And with all the attention surrounding Toyota and other automobile manufacturers for sudden-acceleration issues that may or may not be related to electronics problems, it's worthwhile pointing out some of the steps motorcycle manufacturers take to ensure that a failure in an electronic system or rider aid does not cause an accident. As important as this aspect is for automobile manufactures, I'd suspect it's even more important for a motorcycle company because a failure of some sort is more likely to lead to an accident, and that accident in turn is more likely to lead to an injury.
One of the biggest concerns in the automotive industry right now is unintended acceleration, or sticking throttles. This is what Toyota has been under such close scrutiny for, but some models from Ford and Chrysler have also recently come under the microscope for the same problem according to articles in the Los Angeles Times. On a bike with ride-by-wire throttle, it's not inconceivable that a glitch in the electronics could cause the throttle to stay open-or worse, to open itself. Thankfully, this scenario is highly unlikely thanks to the use of multiple redundancies. Since the early eighties most motorcycles have used two throttle cables, one for opening and one for closing the carburetors or throttle bodies. This is in addition to a spring that on its own is capable of closing the throttle. The Department of Transportation requires that the throttle be "self-closing to idle in a clockwise direction after release of hand", but the closing throttle cable is not a requirement-the manufacturers have added that feature on their own initiative.
When it comes to ride-by-wire throttle, things get a little complicated. The ECU controls the throttle bodies' butterflies, and the implication is that the throttle itself has no physical connection. But on the Yamaha YZF-R6, an interesting arrangement between the servomotor working the butterflies and the throttle cables means the closing cable still physically closes the butterflies and won't allow them to open more than the throttle is opened. I haven't had a chance to look closely at either the Honda VFR1200F or BMW S 1000 RR ride-by-wire setups, but the proximity of the throttle cam to the butterflies and servomotors on both models leads me to believe there is still some form of physical connection there. And I'm certain that other ride-by-wire models offered in the future will be likewise equipped.
One safety feature that some automobiles have is a brake-override switch, which automatically closes the throttle if the brake pedal is applied. This may or may not be mandated for all cars in the future, and could easily be incorporated into a motorcycle's ride-by-wire setup if necessary. An unfortunate byproduct of this is that you wouldn't be able to blip the throttle for a downshift (if you were applying the brakes at the same time). Perhaps worse, it would be difficult to leave a stop on an uphill if you couldn't have at least some power with the brakes still applied. It will be interesting to see what transpires if this feature is incorporated into a motorcycle model in the future.
Even with closing cables and stiff return springs, I've had throttles stick occasionally. On one of my first tests of a racebike for Cycle Canada, the bike's throttle stuck wide open on my second lap, which was a bit of a pucker moment. And when I first moved to California and had to re-take my motorcycle riding test at the DMV, I couldn't even do a U-turn on the bike I had intended to use as the engine revved up whenever the bars were turned, thanks to a misrouted throttle cable (my own fault). If the throttle on a bike ever does happen to stick open, there are several options for avoiding trouble on a motorcycle that some modern cars simply don't have, such as pulling in the clutch or turning the key off. Motorcycles are also required by the DOT to have a "supplemental engine-stop control" on the right handlebar (the kill switch), and this adds yet another level of insurance.
The other big rider aid that is a bit worrisome from a failure standpoint is ABS or electrically assisted brakes. After all, an ABS system is designed to prevent you from applying the brakes as much as you intend to, and if an electric braking system loses power you could end up with no brakes. Again, there are redundant systems in place to prevent any such problems. The DOT requires that a motorcycle must have two independently actuated brake systems or a "split-service brake system" such as any of the linked or combined setups. Those systems must be designed such that a pressure failure in the front brake does not affect the operation of the rear brake, and vice versa. Motorcycles equipped with electrically assisted brakes are not required to meet the same safety standard that similarly equipped automobiles must comply with, which mandates that in the event of an electrical problem or failure, the system will still work. Again, the motorcycle manufacturers have made this step on their own, and Hondas with Combined ABS brakes and BMWs with servo-assisted ABS brakes still stop when the key is turned off or if there is a problem. That's not to say these fail-safes are idiot-proof. The first BMWs with the servo-assist required a lot of lever effort to work the brakes without power, something I found out when trying to back a big R1200T down our dyno's ramp. I almost ended up in a heap at the bottom when I pulled the lever as usual and nothing happened. Thankfully, the newer versions of this system are much improved.
So, the answer to the question at the top of the page is that it's highly unlikely your ride-by-wire-equipped bike will suddenly accelerate on its own. And it's nice to know that the motorcycle manufacturers are well aware of these issues and designing features using new technology accordingly.