Is it really necessary for Twitter to put my cell phone number and e-mail address in the HTML source code, in the clear?
To see your cell phone number and e-mail address, log into your Twitter account and then view an individual tweet. For example, view this tweet's source code and search for "Init CurrentUser Method":
Usually, sensitive information, like this, is encoded in a cookie. The cookie can change the information encoded, from time to time (for example, each time you log in). However, I can't change my cell phone number if it's compromised.