If you tap the voice button immediately after typing a letter, the device registers a space instead of activating voice. I assume this is Apples attempt to reduce inadvertent touches. Open up notes and try it. Type a bunch of letters and then quickly tap the voice button (at roughly the same speed you'd normally tap the space bar). Notice that it registers a space instead of the voice, even though you tapped right over the voice button. If you wait a second and then press the same spot, voice will now activate. Pretty clever on Apples part.
Although I could see this being a problem. Fortunately I tend to naturally tap the space bar with my right thumb so it's difficult to miss, but if you do it with your left thumb I can see it being a problem at times.