PSTN systems have been in use for a long enough time that they've managed to integrate with all other systems including law enforcement. Children are taught early on that "911" is the number to call in an emergency. All regular phones are subscribed to this service. It relies on the authorities being able to track you based on the telephone number you call from since those numbers always correspond to a physical location.
VoIP however, has unique challenges in this area. Because of the nature of the technology, the only identifiable piece of information available is the IP address. Though IP addresses can be tied to a location, they usually aren't. Broadband providers very often assign IP addresses dynamically meaning each time you log on to the Internet you have a different IP address. The problem is exacerbated with mobile Internet users who may be using the services of another provider. It's pretty clear that it's quite impossible to easily find out where a VoIP user is located.
911 and other emergency calls depend on knowing where the user is. To start off with, the call needs to be routed to the closest Public Safety Answering Point (PSAP) - no use in having the calltaker in another state for example. Apart from that, the answerer needs to know exactly where the person is. In the heat of the moment, it can be very difficult to extract an address from a person.
To cope with this problem, US VoIP providers are required to provide a service called Enhanced 911 or E911. This service is optional and VoIP firms are required to let their users know that there are still issues with emergency calls over VoiP. One of the ways this is implemented is to collect the data manually from the subscribers and hope that they keep the information up to date. By law, the burden is on the subscribers to do this and not the VoIP company.
This is an evolving issue. In 2008, a law was passed which gave VoIP providers the right to connect to the 911 system over the trunk owned by traditional carriers. Co operation in this area is hampered by the fact that established telcos don't want VoIP as a competitor and aren't very cooperative in general.
As a VoIP user, make sure that you give your provider a physical address to associate with emergency calls. If you do that, those answering your call will know where to go if you can't provide them with the information immediately.