Twitter update:

I emailed Twitter's support address asking about privacy issues. They replied, I asked for an explicit clarification, and they replied with that too. Here's the text of the emails:

> Some people I've shown the site to have asked whether they should be
> worried about giving you their phone number.  At a quick glance, I'm not
> sure what to tell them - you don't have anything I can see which
> promises not to send my friends SMS spam for ever and a day if they sign
> up.  It would be nice to see a privacy policy appear next to the ToS link...

Thanks for your email.  We're working on a privacy policy now.  In the meantime, 
thanks for your patience, and we will not give out any one's personal information.

> Specifically, can you confirm that you won't be using or selling the
> phone numbers you're gathering for any purpose other than sending
> messages generated by the user's Twitter friends?

We absolutely will not sell phone numbers!  The only reason we ask for phone 
numbers is to deliver the text messages to you, and only the ones you opt in 
to receive.  We should have a privacy policy up soon, but in the interim, if 
you feel uncomfortable having your phone number online, you can always use 
Twitter with IM or on the web only.

So... go sign up! :)