I have seen a disturbing trend where websites are starting to require verification sent to cellphones by text message (SMS). Gmail and Facebook are two of them. What I want to know are the following:

1. Is it a good idea to start requiring cellphones instead of emails now?
2. How do I do it on my own website?

Edit

Here are some of my new questions on the topic in response to the answers:

1. I see that most of you are saying that SMS registrations is ok. But what about the people who don't have cell phones? And why is it accepted to give out your cell phone information freely?
2. Do those big providers really pay per message to a gateway service? Is it not possible to set up a server with the correct SMS software, or at least buy a subscription directly instead of having a middleman?