This was just forwarded to me via email from a friend, don't have access to the original article.

Consumers Reports, Jan 2003 edition.

What's more, consumers who purchase cell phones for security in an emergency
often are shortchanged, he said. In a separate survey of its Web site
subscribers, Consumer Reports found that 15 percent of nearly 1,900 people
who tried to call 911 from a cell phone in the previous year said they had
trouble connecting. Four percent reported never getting through at all.
Nine of 21 test calls failed to connect with emergency dispatchers even
though a strong signal was available from a competitor, Consumers Union
Senior Vice-President for Technical Policy R. David Pittle said.
Those who buy phones with the ability to switch between analog and digital
often find more reliable 911 service, he said. "Phones that work in both of
these modes offer you more chances to get through in an emergency."