We had known this was coming, but as of today you'll be able to get in contact with emergency services by texting 911. This is the "Text-to-911" setup and for now there is support coming from the four major US carriers to include AT&T, T-Mobile, Verizon Wireless and Sprint. For now though -- support on the 911 side is still a bit limited.
In a FCC document dated May 9, 2014, there is support listed for select cities across 16 states. The full breakdown of the city and state support can be seen in this "Text-to-911 Deployments" report. Support is expected to grow in the coming months, however there is a bit of good news for those attempting to text in a city that isn't yet supported.
The FCC has bounce-back messages setup so you'll be notified if Text-to-911 isn't available. In that case you would then have to actually make a call to 911 to get emergency services. The FCC is also still recommending those in trouble to "always contact 911 by making a voice call, if you can." They have also made it clear that "in most cases now, you cannot reach 911 by sending a text message."
We suggest those planning (hoping) to use this setup check the "Text-to-911 Deployments" report ahead of time. After all, while the bounce-back message should be quick, we suspect you will not want any sort of delay if the situation truly is an emergency. Bottom line here, while support for Text-to-911 is officially available as of today -- the support is still somewhat limited.