Mobile assumptions

As I signed into Google to post on the blog, a screen came up after I typed my password that encouraged me to enter my mobile phone number so that if I became locked out and forgot my password, my cell could be used in some way to get me logged back in. I do not, in fact, have a cell phone. I find it interesting that they have become so ubiquitous in only 30 years or so that the common assumption is that everyone has one. What other technological inventions have similar rates of adoption, I wonder? MP3 players, perhaps?