A lot of homeless people are using cell phones, mostly likely buying "pay-as-you-go" minutes. Those that find a way to sign up for annual plan without a reliable home address sometimes have problems paying the bills, and this can impact their credit record (which can impact applications for low-income housing, etc.). The cheap plans don't always include voice mail. And then there is the issue of having a reliable place to recharge your phone.
Ditto for email and the Web. There are obviously a lot of free access points, and a lot of information on the Web that can help people help themselves. If every homeless person had an email account, it would make it quite a bit easier to provide them with services (e.g. a broadcast message each week about job/housing opportunities in a particular city). As it is, we do this with voice mail, and it works reasonably well.
We're conducting focus groups and survey research on both topics this summer, with voice mail users in about five cities. Happy to post a follow-up about this when we have our data. (I'll leave the discussion about why and how people are homeless to those with more knowledge than I. I spend all my time trying to give people this voice mail tool so they can become ex-homeless!).