This morning I finally got around to configuring my twitter account that I created way back when it launched. I tend to create accounts on everything in beta. And, play around with them for a few weeks, get bored and only eventually wander back (usually only if it becomes popular useful).
I was quite impressed with the twitter mobile phone verification letter pattern. Instead of the typical engineering hash of crap, the engineers or product managers at twitter make the alpha-code to correspond with the first letter on a typical phone number pad. That is, they have a 6 letter code that is a combination of: a, d, g, j, m, p, t, w. This fact prevented me from having to click a number on my phone more than once. Ah, the little things that make a big difference in customers actually opting to verify.
All that said, probably 90% of the twitter users have a smartphone, so this forethought probably doesn’t actually adjust the rate of verification. But, it will as the service continues to grow.